Online Etymology Dictionary

slay (v.): Old English slean "to smite, strike, beat," also "to kill with a weapon, slaughter" (class VI strong verb; past tense sloh, slog, past participle slagen), from Proto-Germanic *slahan, from root *slog- "to hit" (source also of Old Norse and Old Frisian sla, Danish slaa, Middle Dutch slaen, Dutch slaan, Old High German slahan, German schlagen, Gothic slahan "to strike"). The Germanic words are from PIE root *slak- "to strike" (source also of Middle Irish past participle slactha "struck," slacc "sword").

Modern German cognate schlagen maintains the original sense of "to strike." Meaning "overwhelm with delight" (mid-14c.) preserves one of the wide range of meanings the word once had, including, in Old English, "stamp (coins); forge (weapons); throw, cast; pitch (a tent), to sting (of a snake); to dash, rush, come quickly; play (the harp); gain by conquest."
slay (n.): "instrument on a weaver's loom to beat up the weft," Old English slæ, slea, slahae, from root meaning "strike" (see slay (v.)), so called from "striking" the web together. Hence the surname Slaymaker "maker of slays."
slayer (n.): late 14c., agent noun from slay (v.). The Old English agent noun was slaga "slayer, killer."
sleave (v.): "to separate or divide" (threads, strands, fibers), Old English -slæfan, from stem of -slifan "to separate, split, cleave," from Proto-Germanic *slifanan, perhaps related to the root of slip (v.). Compare German Schleife "a loop, knot, noose." Related: Sleaved; sleaving. As a noun, "knotted, tangled silk or thread," 1590s, from the verb; this is the word in Shakespeare's rauel'd Sleeue of Care ("Macbeth").
sleaze (n.): "condition of squalor," by 1967, back-formation from sleazy. Meaning "person of low moral standards," and the adjectival use, are attested from 1976.
sleazy (adj.): 1640s, "downy, fuzzy," later "flimsy, unsubstantial" (1660s), of unknown origin; one theory is that it is a corruption of Silesia, the German region, where thin linen or cotton fabric was made for export. Silesia in reference to cloth is attested in English from 1670s; and sleazy as an abbreviated form is attested from 1670), but OED is against this. Sense of "sordid" is from 1941. Related: Sleazily; sleaziness.
A day is a more magnificent cloth than any muslin, the mechanism that makes it is infinitely cunninger, and you shall not conceal the sleazy, fraudulent, rotten hours you have slipped into the piece, nor fear that any honest thread, or straighter steel, or more inflexible shaft, will not testify in the web. [Emerson, "The Conduct of Life," 1860]
sled (n.): early 14c., "a dragged vehicle used for transport of heavy goods," from Middle Dutch sledde "sled," from Proto-Germanic *slid- (source also of Old Saxon slido, Old Norse sleði, Danish slæde, Swedish släde, Old High German slito, German Schlitten "sledge"), from the same root as Old English slidan (see slide (v.)). Not found in Old English. In reference to a sleigh used for travel or recreation, it is attested from 1580s, now mainly American English.
sled (v.): "transport on a sled," 1718; "ride on a sled," 1780, from sled (n.). Related: Sledded; sledding.
sledge (n.1): "heavy hammer," Old English slecg "hammer, mallet," from Proto-Germanic *slagjo- (source also of Old Norse sleggja, Middle Swedish sleggia "sledgehammer"), related to slege "beating, blow, stroke" and slean "to strike" (see slay (v.)). Sledgehammer is pleonastic.
sledge (n.2): "sleigh," 1610s, from dialectal Dutch sleedse, variant of slede (see sled (n.)); said by OED to be perhaps of Frisian origin.
sledgehammer (n.): late 15c., from sledge (n.1) + hammer (n.). As a verb, from 1834. Old English had slegebytel "hammer," from beetle (n.2).
sleek (adj.): 1580s, variant of Middle English slike (see slick (adj.)). Originally of healthy-looking animal hair; applied to persons 1630s, with sense of "plump and smooth-skinned." Figurative meaning "slick, fawning, flattering" is from 1590s.
sleek (v.): "make sleek," mid-15c., a variant of slick (v.). Related: Sleeked; sleeking.
sleep (n.): Old English slæp "sleep, sleepiness, inactivity," from Proto-Germanic *slepaz, from the root of sleep (v.); compare cognate Old Saxon slap, Old Frisian slep, Middle Dutch slæp, Dutch slaap, Old High German slaf, German Schlaf, Gothic sleps.

Personified in English from late 14c., on model of Latin Somnus), Greek Hypnos. Figurative use for "repose of death" was in Old English; to put (an animal) to sleep "kill painlessly" is recorded from 1923 (a similar imagery is in cemetery). Sleep deprivation attested from 1906. Sleep-walker "somnambulist" is attested from 1747; sleep-walking is from 1840. To be able to do something in (one's) sleep "easily" is recorded from 1953.
sleep (v.): Old English slæpan "to be or fall asleep; be dormant or inactive" (class VII strong verb; past tense slep, past participle slæpen), from Proto-Germanic *slepan (source also of Old Saxon slapan, Old Frisian slepa, Middle Dutch slapen, Dutch slapen, Old High German slafen, German schlafen, Gothic slepan "to sleep"), from PIE root *sleb- "to be weak, sleep" (source also of Old Church Slavonic slabu "lax, weak," Lithuanian silpnas "weak"), which perhaps is connected to the root of slack (adj.). Sleep with "do the sex act with" is in Old English:
Gif hwa fæmnan beswice unbeweddode, and hire mid slæpe ... [Laws of King Alfred, c.900]
Related: Slept; sleeping. Sleep around first attested 1928.
sleep-over (n.): 1935, from verbal phrase; see sleep (v.) + over (adv.).
sleeper (n.): Old English slæpere "one who sleeps, one who is inclined to sleep much," agent noun from sleep (v.). Meaning "strong horizontal beam" is from c. 1600. Meaning "dormant or inoperative thing" is from 1620s. Meaning "railroad sleeping car" is from 1875. Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in American English sports jargon, probably from earlier (1856) gambling slang sense of "unexpected winning card." Meaning "spy, enemy agent, terrorist etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in reference to communist agents in the West.
sleeping (adj.): c. 1300, past participle adjective from sleep (v.). Sleeping-pill is from 1660s; sleeping-bag is from 1850; sleeping sickness as a specific African tropical disease is first recorded 1875; sleeping has been used since late 14c. for diseases marked by morbid conditions. Sleeping Beauty (1729) is Perrault's La belle au bois dormant.
It is ill wakyng of a sleapyng dogge. [Heywood, 1562]

It is nought good a slepyng hound to wake. [Chaucer, c. 1385]
sleepless (adj.): early 15c., from sleep (n.) + -less. Old English had slæpleas but the modern word seems to be a re-formation. Similar formation in German schlaflos, Dutch slapeloos. Related: Sleeplessly; sleeplessness.
sleepy (adj.): early 13c. from sleep (n.) + -y (2). Perhaps in Old English but not recorded. Old English had slæpor, slæpwerig in the sense "sleepy;" slæpnes "sleepiness." Similar formation in Old High German slafag. Of places, from 1851 (Irving's Sleepy Hollow is from 1820). Sleepy-head is from 1570s. Related: Sleepily; sleepiness.
sleet (n.): c. 1300, slete, either from an unrecorded Old English *slete, *slyte, related to Middle High German sloz, Middle Low German sloten (plural) "hail," from Proto-Germanic *slautjan- (source also of dialectal Norwegian slutr, Danish slud, Swedish sloud "sleet"), from root *slaut-.
sleet (v.): early 14c., from sleet (n.). Related: Sleeted; sleeting.
sleeve (n.): Old English sliefe (West Saxon), slefe (Mercian) "arm-covering part of a garment," probably literally "that into which the arm slips," from Proto-Germanic *slaubjon (source also of Middle Low German sloven "to dress carelessly," Old High German sloufen "to put on or off"). Related to Old English slefan, sliefan "to slip on (clothes)" and slupan "to slip, glide," from PIE root *sleubh- "to slide, slip."

Compare slipper, Old English slefescoh "slipper," slip (n.2) "woman's garment," and expression slip into "dress in." Mechanical sense is attested from 1864. Meaning "the English Channel" translates French La Manche. To have something up one's sleeve is recorded from c. 1500 (large sleeves formerly doubled as pockets). To wear one's heart on (one's) sleeve is from "Othello" (1604).
sleeveless (adj.): Old English sliefleas, slyflease; see sleeve (n.) + -less.
sleigh (n.): "vehicle mounted on runners for use on ice and snow," 1703, American and Canadian English, from Dutch slee, shortened from slede (see sled (n.)). As a verb from 1728. Related: Sleighing. Sleigh-ride is first recorded 1770; sleigh-bells is from c. 1780; they originally were used to give warning of the approach of a sleigh.
sleight (n.): "cunning," early 14c. alteration of sleahthe (c. 1200), from Old Norse sloegð "cleverness, cunning, slyness," from sloegr (see sly). Meaning "skill, cleverness, dexterity" is from late 14c. Meaning "feat or trick requiring quickness and nimbleness of the hands" is from 1590s. Term sleight of hand is attested from c. 1400.
slender (adj.): c. 1400, earlier sclendre (late 14c.), probably from a French source, often said to be from Old French esclendre "thin, slender," which could be from Old Dutch slinder, but the connections, and even the existence of these words, is doubtful. Related: Slenderly; slenderness.
slenderize (v.): 1921, from slender + -ize. Related: Slenderized; slenderizing. As a verb, slender "make narrower" is from 1590s.
slept: past tense and past participle of sleep (v.).
sleuth (n.): c. 1200, "track or trail of a person," from Old Norse sloð "trail," of uncertain origin. Meaning "detective" is 1872, shortening of sleuth-hound "keen investigator" (1849), a figurative use of a word that dates back to late 14c. meaning a kind of bloodhound. The verb (intransitive) meaning "to act as a detective, investigate" is recorded from 1905. Related: Sleuthed; sleuthing.
slew (n.1): "swampy place," 1708, North American variant of slough.
slew (v.): "to turn, swing, twist," 1834, earlier slue (1769), a nautical word, of unknown origin. Slewed (1801) is old nautical slang for "drunk." Slew-foot "clumsy person who walks with feet turned out" is from 1896.
slew (n.2): "large number," 1839, from Irish sluagh "a host, crowd, multitude," from Celtic and Balto-Slavic *sloug- "help, service" (see slogan).
slice (v.): late 15c., from Middle French esclicier, from Old French escliz (see slice (n.)). Golfing sense is from 1890. Related: Sliced; slicing. Sliced bread is attested from 1929 and was touted in advertisements; greatest thing since ... first attested 1969.
With the advent of ready sliced bread the bread board, the bread knife and the slicing machine pass out of the picture. Sliced bread is a radical departure in the baking industry and although the Weber Baking Company will continue to supply the trade with unsliced loaves, the company anticipates an unusual run on the ready sliced loaf. ["Western Hospital Review," Volume 14, 1929]

No matter how thick or how thin you slice it it's still baloney. [Carl Sandburg, "The People, Yes," 1936]
slice (n.): c. 1300, "a fragment," from Old French escliz "splinter, fragment" (Modern French éclisse), a back-formation from esclicier "to splinter, shatter, smash," from Frankish *slitan "to split" or some other Germanic source (compare Old High German slihhan; see slit (v.)). Meaning "piece cut from something" emerged early 15c. Meaning "a slicing stroke" (in golf, tennis) is recorded from 1886. Slice of life (1895) translates French tranche de la vie, a term from French Naturalist literature.
slicer (n.): 1520s, agent noun from slice (v.).
slick (n.): 1620s, a kind of cosmetic, from slick (v.). Meaning "smooth place on the surface of water caused by oil, etc." is attested from 1849. Meaning "a swindler, clever person" is attested from 1959.
slick (v.): Old English -slician (in nigslicod "newly made sleek"), from Proto-Germanic *slikojan, from base *slikaz (source also of Old Norse slikr "smooth," Old High German slihhan "to glide," German schleichen "to creep, crawl, sneak," Dutch slijk "mud, mire"), from PIE *sleig- "to smooth, glide, be muddy," from root *(s)lei- "slimy" (see slime (n.)). Related: Slicked; slicking.
slick (adj.): early 14c., "smooth, glossy, sleek" (of skin or hair); sense of "clever in deception" is first recorded 1590s; that of "first-class, excellent" is from 1833. Related: Slickly; slickness.
slicker (n.): 1851, "tool for smoothing leather," from slick (v.). Meaning "waterproof raincoat" is from 1884; sense of "clever and crafty person" is from 1900.
slid: past tense and past participle of slide (v.).
slide (n.): 1560s, from slide (v.). As a smooth inclined surface down which something can be slid, from 1680s; the playground slide is from 1890. Meaning "collapse of a hillside, landslide" is from 1660s. As a working part of a musical instrument from 1800 (as in slide-trombone, 1891). Meaning "rapid downturn" is from 1884. Meaning "picture prepared for use with a projector" is from 1819 (in reference to magic lanterns). Baseball sense is from 1886. Slide-guitar is from 1968.
slide (v.): Old English slidan (intransitive, past tense slad, past participle sliden) "to glide, slip, fall, fall down;" figuratively "fail, lapse morally, err; be transitory or unstable," from Proto-Germanic *slidan "to slip, slide" (source also of Old High German slito, German Schlitten "sleigh, sled"), from PIE root *sleidh- "to slide, slip" (source also of Lithuanian slystu "to glide, slide," Old Church Slavonic sledu "track," Greek olisthos "slipperiness," olisthanein "to slip," Middle Irish sloet "slide").

Meaning "slip, lose one's footing" is from early 13c. Transitive sense from 1530s. Phrase let (something) slide "let it take its own course, take no consideration of" is in Chaucer (late 14c.) and Shakespeare. Sliding scale in reference to payments, etc., is from 1842.
slide-rule (n.): also slide rule, mathematical calculating tool, 1838, from slide (v.) + rule (n.). So called for its method of operation. Earlier sliding-rule (1660s).
slider (n.): 1520s, "skater," agent noun from slide (v.). As a type of terrapin, from 1877; as a type of baseball pitch, 1936.
slight (adj.): early 14c., "flat, smooth; hairless," probably from a Scandinavian source akin to Old Norse slettr "smooth, sleek," from Proto-Germanic *slikhtaz (source also of Old Saxon slicht; Low German slicht "smooth, plain common;" Old English -sliht "level," attested in eorðslihtes "level with the ground;" Old Frisian sliucht "smooth, slight," Middle Dutch sleht "even, plain," Old High German sleht, Gothic slaihts "smooth"), probably from a collateral form of PIE *sleig- "to smooth, glide, be muddy," from root *(s)lei- "slimy" (see slime (n.)).

Sense evolution probably is from "smooth" (c. 1300), to "slim, slender; of light texture," hence "not good or strong; insubstantial, trifling, inferior, insignificant" (early 14c.). Meaning "small in amount" is from 1520s. Sense of German cognate schlecht developed from "smooth, plain, simple" to "bad, mean, base," and as it did it was replaced in the original senses by schlicht, a back-formation from schlichten "to smooth, to plane," a derivative of schlecht in the old sense [Klein].
slight (v.): c. 1300, "make plain or smooth," from slight (adj.) Meaning "treat with indifference" (1590s) is from the adjective in sense of "having little worth." Related: Slighted; slighting.
slight (n.): 1550s, "small amount or weight," from slight (v.). Meaning "act of intentional neglect or ignoring out of displeasure or contempt" is from 1701, probably via 17c. phrase make a slight of (1610s).
slightly (adv.): 1520s, "slenderly;" 1590s, "in a small degree," from slight (adj.) + -ly (2).
slim (adj.): 1650s, "thin, slight, slender," from Dutch slim "bad, sly, clever," from Middle Dutch slim "bad, crooked," from Proto-Germanic *slembaz "oblique, crooked" (source also of Middle High German slimp "slanting, awry," German schlimm "bad, cunning, unwell"). In English 17c. also sometimes with a sense "sly, cunning, crafty." Related: Slimly; slimness. With obsolete extended adjectival forms Slimsy "flimsy, unsubstantial" (1845); slimikin "small and slender" (1745). Slim Jim attested from 1887 in sense of "very thin person;" from 1902 as a type of slender cigar; from 1975 as a brand of meat snack.