Online Etymology Dictionary

sign-in (n.): 1968, from the verbal phrase; see sign (v.) + in (adv.).
sign-up (n.): "number who have signed up," 1926, from the verbal phrase; see sign (v.) + up (adv.).
signage (n.): 1972, in reference to signs on roads or outside stores, from sign (n.) + -age. Earlier in legal language with reference to affixing signatures.
signal (adj.): "remarkable, striking, notable" ("serving as a sign"), 1640s, from French signalé, past participle of signaler "to distinguish, signal" (see signal (n.)).
signal (n.): late 14c., "visible sign, indication," from Old French signal, seignal "seal, imprint, sign, mark," from Medieval Latin signale "a signal," from Late Latin signalis (adj.) "used as a signal, pertaining to a sign," from Latin signum "signal, sign" (see sign (n.)). Restricted sense "agreed-upon sign (to commence or desist, etc.) is from 1590s. Meaning "modulation of an electric current" is from 1855.
signal (v.): 1805, "to make signals to," from signal (n.). Related: Signaled; signaling. Earlier verb was signalize (1650s).
signatory (adj.): 1640s, "used in sealing," from Latin signatorius "of sealing," from signatus, past participle of signare "to sign" (see sign (v.)). Noun sense of "one who signs" (a treaty, etc.) first recorded 1866.
signature (n.): 1530s, a kind of document in Scottish law, from Middle French signature (16c.) or directly from Medieval Latin signatura "signature, a rescript," in classical Latin "the matrix of a seal," from signatus, past participle of signare "to mark with a stamp, sign" (see sign (v.)).

Meaning "one's own name written in one's own hand" is from 1570s, replacing sign-manual (early 15c.) in this sense. Musical sense of "signs placed it the beginning of a staff to indicate the key and rhythm" is from 1806. Meaning "a distinguishing mark of any kind" is from 1620s.
signboard (n.): also sign-board, 1630s, from sign (n.) + board (n.1).
signee (n.): 1953, from sign (v.) + -ee.
signer (n.): 1610s, agent noun from sign (v.). In American history, with reference to the Declaration of Independence, by 1865.
signet (n.): late 14c., "small seal" (especially one on a finger ring), from Old French signet "a small seal," diminutive of signe "sign" (see sign (n.)).
significance (n.): c. 1400, "meaning," from Old French significance or directly from Latin significantia "meaning, force, energy," from significans, present participle of significare "to mean, import, signify" (see signify). The earlier word was signifiance (mid-13c.). Meaning "importance" is from 1725. Related: Significancy.
significant (adj.): 1570s, "having a meaning," from Latin significantem (nominative significans, present participle of significare "make known, indicate" (see signify). Earlier in the same sense was significative (c. 1400). Often "having a special or secret meaning," hence "important" (1761). Related: Significantly. Significant figure is from 1680s. Significant other (n.) attested by 1961, in psychology, "the most influential other person in the patient's world."
signification (n.): early 14c., "symbolization, representation," from Old French significacion and directly from Latin significationem (nominative significatio) "a signifying, indication, expression, sign, token, meaning, emphasis," noun of action from past participle stem of significare "make known, indicate" (see signify). From late 14c. as "meaning" (of a word, etc.).
signifier (n.): 1530s, agent noun from signify. In African-American vernacular use by 1962.
signify (v.): late 13c., "be a sign of, indicate, mean," from Old French signifier (12c.), from Latin significare "to make signs, show by signs, point out, express; mean, signify; foreshadow, portend," from significus (adj.), from signum "sign" (see sign (n.)) + root of facere "to make" (see factitious). Intransitive sense of "to be of importance" is attested from 1660s. Meaning "engage in mock-hostile banter" is African-American vernacular, by 1932.
...'signifying,' which in Harlemese means making a series of oblique remarks apparently addressed to no one in particular, but unmistakable in intention in such a close-knit circle. ["Down Beat," March 7, 1968]
signor (n.): an Italian lord or gentleman, 1570s, from Italian signore, from Latin seniorem, accusative of senior (see senior (adj.)). Feminine form signora is from 1630s; diminutive signorina is first recorded 1820.
signpost (n.): also sign-post, 1610s, "sign on a post, usually indicating an inn or shop," from sign (n.) + post (n.1). Meaning "guide- or direction-post along a road" is attested from 1863. Figurative sense is from 1889.
sike (n.): also syke, "small stream," a Scottish and Northern word, from Old English sic or cognate Old Norse sik "a ditch, trench."
Sikh (n.): 1781, member of a politico-religious community established c. 1500 in Punjab by Nanak Shah, from Hindi sikh "disciple," from Sanskrit siksati "studies, learns," related to saknoti "he is able, he is strong" (see Shakti).
Sikhism (n.): "tenets of the Sikhs," 1849, from Sikh + -ism.
silage (n.): "fodder packed in a silo," 1884, alteration (probably by influence of silo) of ensilage.
Silas: masc. proper name, from Late Latin, from Greek Silas, contraction of Silouanos, transliteration of Latin Silvanus, a name that literally means "living in the woods," from silva "wood" (see sylvan).
silence (n.): c. 1200, "muteness, state of being silent," from Old French silence "state of being silent; absence of sound," from Latin silentium "a being silent," from silens, present participle of silere "be quiet or still," of unknown origin. Meaning "absence of sound" in English is from late 14c.
silence (v.): 1560s, intransitive, "become still or silent;" 1590s, transitive, "make silent," from silence (n.). Related: Silenced; silencing.
silencer (n.): c. 1600, "person who silences," agent noun from silence (v.). Meaning "mechanism that stifles the sound of a motor or firearm" is from 1898.
silent (adj.): c. 1500, "without speech, silent, not speaking," from Latin silentem (nominative silens) "still, calm, quiet," present participle of silere "be quiet or still" (see silence (n.)). Meaning "free from noise or sound" is from 1580s.

Of letters, c. 1600; of films, 1914. In the looser sense "of few words," from 1840. Phrase strong, silent (type) is attested from 1905. Silent majority in the political sense of "mass of people whose moderate views are not publicly expressed and thus overlooked" is first attested 1955 in a British context and was used by John F. Kennedy but is most associated in U.S. with the rhetoric of the Nixon administration (1969-74).
It is time for America's silent majority to stand up for its rights, and let us remember the American majority includes every minority. America's silent majority is bewildered by irrational protest. [Spiro T. Agnew, May 9, 1969]
In Victorian use, the phrase meant "the dead" (1874; compare Roman use of the noun plural of "silent" to mean "the dead"). Silence is golden (1831) is Carlyle's translation ["Sartor Resartus"] of part of the "Swiss Inscription" Sprechen ist silbern, Schweigen ist golden. In one 14c. text Latin "one who is silent" is translated by a beere stille.
silently (adv.): 1560s, from silent (adj.) + -ly (2).
Silenus: 1710, from Greek Seilenos, foster-father of Bacchus and leader of the satyrs; the name is of unknown origin (Klein compares Thracian zilai "wine").
Silesia: former eastern German province, now southwestern Poland, from Latinized form of German Schliesen (Polish Śląsk), from the name of a river and a mountain there, from Silingi or Silingae, name of a Vandalic (Germanic) people who supposedly had a religious center at the mountain. Related: Silesian. In reference to cloth imported from there from 1670s, especially "a thin cotton cloth, commonly twilled, used for linings for women's dresses and men's garments."
silhouette (n.): 1798, from French silhouette, in reference to Étienne de Silhouette (1709-1767), French minister of finance in 1759. Usually said to be so called because it was an inexpensive way of making a likeness of someone, a derisive reference to Silhouette's petty economies to finance the Seven Years' War, which were unpopular among the nobility. But other theories are that it refers to his brief tenure in office, or the story that he decorated his chateau with such portraits.
Silhouette portraits were so called simply because they came into fashion in the year (1759) in which M. de Silhouette was minister. [A. Brachet, "An Etymological Dictionary of the French Language," transl. G.W. Kitchin, 1882]
Used of any sort of dark outline or shadow in profile from 1843. The verb is recorded from 1876, from the noun. The family name is a Frenchified form of a Basque surname; Arnaud de Silhouette, the finance minister's father, was from Biarritz in the French Basque country; the southern Basque form of the name would be Zuloeta or Zulueta, which contains the suffix -eta "abundance of" and zulo "hole" (possibly here meaning "cave").
silica (n.): "hard silicon dioxide," 1801, Modern Latin, from Latin silex (genitive silicis) "flint, pebble," on model of alumina, soda.
silicate (n.): 1811, from silica + -ate (3).
silicon (n.): nonmetallic element, 1817, coined by British chemist Thomas Thomson from silica (silicon dioxide), from which it was isolated. The name is patterned on carbon, etc. Silicon chip first attested 1965; Silicon Valley for the Santa Clara Valley south of San Francisco, U.S., first attested 1974, from the concentration of manufacturers of silicon chips used in computers, watches, etc.
silicone (n.): coined 1863 in German from silico-, comb. form indicating the presence of silicon, + -one.
silicosis (n.): 1881, a hybrid from silicon + -osis.
silk (n.): c. 1300, from Old English seoloc, sioloc "silk, silken cloth," from Latin sericum "silk," plural serica "silken garments, silks," literally "Seric stuff," neuter of Sericus, from Greek Serikos "silken; pertaining to the Seres," an oriental people of Asia from whom the Greeks got silks. Western cultivation began 552 C.E., when agents from Byzantium impersonating monks smuggled silkworms and mulberry leaves out of China. Chinese si "silk," Manchurian sirghe, Mongolian sirkek have been compared to this and the people name in Greek might be a rendering via Mongolian of the Chinese word for "silk," but this is uncertain.

Also found in Old Norse as silki but not elsewhere in Germanic. The more common Germanic form is represented by Middle English say, from Old French seie, with Spanish seda, Italian seta, Dutch zijde, German Seide is from Medieval Latin seta "silk," perhaps elliptical for seta serica, or else a particular use of seta "bristle, hair" (see seta (n.)).

According to some sources [Buck, OED], the use of -l- instead of -r- in the Balto-Slavic form of the word (Old Church Slavonic šelku, Lithuanian šilkai) passed into English via the Baltic trade and may reflect a Chinese dialectal form, or a Slavic alteration of the Greek word. But the Slavic linguist Vasmer dismisses that, based on the initial sh- in the Slavic words, and suggests the Slavic words are from Scandinavian rather than the reverse.

As an adjective from mid-14c. In reference to the "hair" of corn, 1660s, American English. Figurative use of silk-stocking (n.) is from 1590s; as an adjective meaning "wealthy" it is attested from 1798, American English (silk stockings, especially worn by men, being regarded as extravagant and reprehensible, indicative of luxurious habits). Silk-screen (n.) is first attested 1930; as a verb from 1961. Silk road so called in English from 1931.
silken (adj.): Old English seolcen "made of silk;" see silk + -en (2). Meaning "silk-like, soft and glossy" is from 1510s.
silkworm (n.): Old English seolcwyrm; see silk + worm (n.).
silky (adj.): 1610s, from silk + -y (2). Related: Silkily; silkiness.
sill (n.): Old English syll "beam, threshold, large timber serving as a foundation of a wall," from Proto-Germanic *suljo (source also of Old Norse svill, Swedish syll, Danish syld "framework of a building," Middle Low German sull, Old High German swelli, German Schwelle "sill"), perhaps from PIE root *swel- (3) "post, board" (source also of Greek selma "beam"). Meaning "lower horizontal part of a window opening" is recorded from early 15c.
sillabub (n.): also sillibub, syllabub, etc., 1530s, of unknown origin. Drink or dish of milk and wine or cider, often sweetened. Figurative sense of "floridly vapid prose" is from 1706.
silliness (n.): "foolishness," c. 1600, from silly + -ness; a reformation of seeliness, from Old English saelignes "happiness, (good) fortune, occurrence."
silly (adj.): Old English gesælig "happy, fortuitous, prosperous" (related to sæl "happiness"), from Proto-Germanic *sæligas (source also of Old Norse sæll "happy," Old Saxon salig, Middle Dutch salich, Old High German salig, German selig "blessed, happy, blissful," Gothic sels "good, kindhearted"), from PIE *sele- "of good mood; to favor," from root *sel- (2) "happy, of good mood; to favor" (source also of Latin solari "to comfort," Greek hilaros "cheerful, gay, merry, joyous").
This is one of the few instances in which an original long e (ee) has become shortened to i. The same change occurs in breeches, and in the American pronunciation of been, with no change in spelling. [Century Dictionary]
The word's considerable sense development moved from "happy" to "blessed" to "pious," to "innocent" (c. 1200), to "harmless," to "pitiable" (late 13c.), "weak" (c. 1300), to "feeble in mind, lacking in reason, foolish" (1570s). Further tendency toward "stunned, dazed as by a blow" (1886) in knocked silly, etc. Silly season in journalism slang is from 1861 (August and September, when newspapers compensate for a lack of hard news by filling up with trivial stories). Silly Putty trademark claims use from July 1949.
It is a widespread phenomenon that the words for 'innocent', apart from their legal use, develop, through 'harmless, guileless', a disparaging sense 'credulous, naive, simple, foolish.' [Buck]
silo (n.): 1835, from Spanish silo, traditionally derived from Latin sirum (nominative sirus), from Greek siros "a pit to keep corn in." "The change from r to l in Spanish is abnormal and Greek siros was a rare foreign term peculiar to regions of Asia Minor and not likely to emerge in Castilian Spain" [Barnhart]. Alternatively, the Spanish word is from a pre-Roman Iberian language word represented by Basque zilo, zulo "dugout, cave or shelter for keeping grain." Meaning "underground housing and launch tube for a guided missile" is attested from 1958.
Siloam: pool and spring outside Jerusalem (John ix:7), from Late Latin, from New Testament Greek, from Hebrew shiloach, literally "sending forth," from shalach "to send."
silphium (n.): plant genus, 1771, from Latin, from Greek Silphion, name of a North African Mediterranean plant whose identity has been lost, the gum or juice of which was prized by the ancients as a condiment and a medicine. Probably of African origin.
silt (n.): mid-15c., originally "sediment deposited by seawater," probably from a Scandinavian source (compare Norwegian and Danish sylt "salt marsh"), or from Middle Low German or Middle Dutch silte, sulte "salt marsh, brine," from Proto-Germanic *sultjo- (source also of Old English sealt, Old High German sulza "saltwater," German Sulze "brine"), from PIE *sal- (see salt (n.)).
silt (v.): "to become choked with silt" (of river channels, harbors, etc.), 1799, from silt (n.). Related: Silted; silting.