Online Etymology Dictionary

lesion (n.): early 15c., "damage, injury," from Old French lesion "hurt, offense, wrong, injury, wound" (12c.), from Latin laesionem (nominative laesio) "a hurting, injuring, personal attack," noun of action from past participle stem of laedere "to strike, hurt, damage," a word of unknown origin with no certain cognates. Originally in English with reference to any sort of hurt, whether physical or not.
less: Old English læs (adv.) "less, lest;" læssa (adj.) "less, smaller, fewer" (Northumbrian leassa), from Proto-Germanic *lais-izo (source also of Old Saxon, Old Frisian les "less;" Middle Dutch lise "soft, gentle," German leise "soft"), from PIE root *leis- (2) "small" (source also of Lithuanian liesas "thin") + comparative suffix.

From the fist the adverb has been used often with negatives (none the less). Much less "still more undesirable" is from 1630s. Formerly also "younger," as a translation of Latin minor, a sense now obsolete except in James the Less. Used as a comparative of little, but not related to it. The noun is Old English læsse.
less (adv.2): early 15c. as a shortening of unless. Extended contraction lessen, less'n, U.S. dialectal, is attested from 1881.
lessee (n.): "one to whom a lease is given," late 15c., from Anglo-French lesee, Old French lessé, past participle of lesser "to let, to leave" (10c., Modern French laisser), from Latin laxare, from laxus "loose" (see lax).
lessen (v.): "to become less," c. 1300, from less (adj.) + -en (1). Transitive sense "to make less" is from c. 1400. Related: Lessened; lessening.
lesser (adj.): early 13c., a double comparative, from less (adj.) + -er (2). Johnson calls it "a barbarous corruption of less, formed by the vulgar from the habit of terminating comparatives in -er." As an adverb from 1590s; now generally poetic or obsolete except in expression lesser-known (1813).
lesson (n.): early 13c., "a reading aloud from the Bible," also "something to be learned by a student," from Old French leçon, from Latin lectionem (nominative lectio) "a reading," noun of action from past participle stem of legere "to read" (see lecture (n.)). Transferred sense of "an occurrence from which something can be learned" is from 1580s.
lessor (n.): "one who grants a lease," late 14c., from Anglo-French lessor (late 13c.), from verb lesser "to let, to leave" (10c., Modern French laisser), from Latin laxare, from laxus "loose" (see lax).
lest (conj.): c. 1200, "that not," especially "for fear that" [OED calls it a negative particle of intention], from a contraction of the Old English phrase þy læs þe "the less that," from þy, instrumental case of demonstrative article þæt "that" + læs (see less) + conjunction þe (see the). The þy was dropped and the remaining two words contracted into early Middle English leste.
let (n.): "stoppage, obstruction" (obsolete unless in legal contracts), late 12c., from archaic verb letten "to hinder," from Old English lettan "hinder, delay, impede, make late," from Proto-Germanic *latjan (source also of Old Saxon lettian "to hinder," Old Norse letja "to hold back," Old High German lezzen "to stop, check," Gothic latjan "to hinder, make late"), related to *lata-, source of late (adj.).
let (v.): Old English lætan (Northumbrian leta) "to allow; to leave behind, depart from; leave undone; bequeath," also "to rent, put to rent or hire" (class VII strong verb; past tense let, leort, past participle gelæten), from Proto-Germanic *letan (source also of Old Saxon latan, Old Frisian leta, Dutch laten, Old High German lazan, German lassen, Gothic letan "to leave, let"), from PIE *led-, extended form of root *le- (2) "to let go, slacken" (source also of Latin lassus "faint, weary," Lithuanian leisti "to let, to let loose;" see lenient). If that derivation is correct, the etymological sense would be "let go through weariness, neglect."

"The shortening of the root vowel ... has not been satisfactorily explained" [OED]. Of blood, from late Old English. Other Old and Middle English senses include "regard as, consider; behave toward; allow to escape; pretend;" to let (someone) know and to let fly (arrows, etc.) preserve the otherwise obsolete sense of "to cause to." To let (someone) off "allow to go unpunished, excuse from service" is from 1814. To let on is from 1725 as "allow (something) to be known, betray one's knowledge of," 1822 as "pretend" (OED finds a similar use in the phrase never let it on him in a letter from 1637). To let out is late 12c. as "allow to depart" (transitive); intransitive use "be concluded," of schools, meetings, etc., is from 1888, considered by Century Dictionary (1902) to be "Rural, U.S." Of garments, etc., late 14c.

Let alone "abstain from interfering with" is in Old English; the phrase in the sense "not to mention, to say nothing of" is from 1812. To let (something) be "leave it alone" is from c. 1300; let it be "let it pass, leave it alone" is from early 14c. To let go is from c. 1300 as "allow to escape," 1520s as "cease to restrain," 1530s as "dismiss from one's thoughts." Let it go "let it pass, no matter" is as old as Chaucer's Wife of Bath: "But age allas Hath me biraft my beautee Lat it go, far wel, the deuel go ther with!" [c. 1395]. Let me see "show me" is from c. 1300.
let-down (n.): also letdown, "a disappointment," 1768, from let (v.) + down (adv.). The verbal phrase is from mid-12c. in a literal sense "cause to be lowered," of drawbridges, etc.; figuratively by 1754.
let-up (n.): "cessation, restraint, relaxation, intermission," 1837, from verbal phrase let up "cease, stop" (1787). In Old English the phrase meant "to put ashore" (let out meant "put to sea"). Bartlett (1848) says the noun is "an expression borrowed from pugilists."
letch (n.): "craving, longing, strong desire," 1796 [Grose], perhaps a back-formation from lecher, or deformed from a figurative use of latch (v.) in a secondary sense of "grasp, grasp on to." Or perhaps from letch (v.), a variant of leach.
lethal (adj.): "causing or resulting in death," 1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from lethum/letum "death," a word of uncertain origin. According to de Vaan, from Proto-Italic *leto-, which is perhaps a noun from a PIE past participle of a verb meaning "let, let go," on the notion of death as "a letting go." If so, related to Old Church Slavonic leto "summer, year" (from notion of "going"), Russian leto "summer," (pl.) "age, years;" Russian let' (archaic) "it is possible, allowed;" Old Norse lað, Old English læð "land," Gothic unleds "poor." The form altered in Late Latin by association with lethes hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness" (see Lethe).
lethality (n.): "deadliness," 1650s, from lethal + -ity.
lethargic (adj.): late 14c., litargik, "morbidly drowsy, manifesting lethargy," from Latin lethargicus "affected with lethargy," from Greek lethargikos "drowsy," from lethargos "forgetful; inactive" (see lethargy). From 1590s as "pertaining to lethargy." Related: Lethargically. In 17c. also with a verb form, lethargize, and a noun, letharge "lethargic patient."
lethargy (n.): late 14c., litarge, "state of prolonged torpor or inactivity, inertness of body or mind," from Medieval Latin litargia, from Late Latin lethargia, from Greek lethargia "forgetfulness," from lethargos "forgetful," apparently etymologically "inactive through forgetfulness," from lethe "a forgetting, forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English. The Medieval Latin word also is the source of Old French litargie (Modern French léthargie), Spanish and Italian letargia.
Lethe: mythical river of Hades (whose water when drunk caused forgetfulness of the past), in Homer, a place of oblivion in the lower world; from Greek lethe, literally "forgetfulness, oblivion," from PIE root *ladh- "be hidden" (see latent). Related to lethargos "forgetful" and cognate with Latin latere "to be hidden." Also the name of a personification of oblivion, a daughter of Eris. Related: Lethean.
Letitia: fem. proper name, literally "gladness," from Latin laetitia "joy, exultation, rejoicing, gladness, pleasure, delight," from laetus "glad, happy; flourishing, rich," a word of unknown origin. On the assumption that "fat, rich" is the older meaning, this word has been connected to lardus "bacon" and largus "generous," but de Vaan finds this "a very artificial reconstruction." In 17c. English had a verb letificate "make joyful" (1620s), and Middle English had letification "action of rejoicing" (late 15c.).
Leto: in Greek mythology, mother of Apollo and Artemis by Zeus. She gave birth to them on the island of Delos. Roman Latona.
Lett (n.): 1831, from German Lette, from Old High German liuti "people" (German Leute), perhaps a German folk-etymologizing of the native name, Latvji (see Latvia). Comb. form Letto-. Related: Lettic (1840); Lettish (1794).
letter (n.2): "one who lets" in any sense, c. 1400, agent noun from let (v.).
letter (v.): "write in letters," 1660s, from letter (n.1). Earlier it was used in a now obsolete sense "instruct" (mid-15c.). Related: Lettered; lettering.
letter (n.1): c. 1200, "graphic symbol, alphabetic sign, written character conveying information about sound in speech," from Old French letre "character, letter; missive, note," in plural, "literature, writing, learning" (10c., Modern French lettre), from Latin littera (also litera) "letter of the alphabet," also "an epistle, writing, document; literature, great books; science, learning;" a word of uncertain origin.

According to Watkins, perhaps via Etruscan from Greek diphthera "tablet" (with change of d- to l- as in lachrymose), from a hypothetical root *deph- "to stamp." In this sense it replaced Old English bocstæf, literally "book staff" (compare German Buchstabe "letter, character," from Old High German buohstab, from Proto-Germanic *bok-staba-m).

Latin littera also meant "a writing, document, record," and in plural litteræ "a letter, epistle, missive communication in writing," a sense passed through French and attested in English letter since early 13c. (replacing Old English ærendgewrit "written message," literally "errand-writing"). The Latin plural also meant "literature, books," and figuratively "learning, liberal education, schooling" (see letters).

The custom of giving the school letter as an achievement award in sports, attested by 1908, is said to have originated with University of Chicago football coach Amos Alonzo Stagg. Earlier in reference to colleges it meant "university degree or honor that adds initials to a name" (1888). Expression to the letter "precisely" is from 1520s (earlier after the letter, mid-14c.). For letters patent (with French word order) see patent (n.).
letter-bag (n.): 1781, from letter (n.1) + bag (n.).
letter-box (n.): 1782, from letter (n.1) + box (n.1).
letter-carrier (n.): 1550s, from letter (n.1) + carrier.
letter-head (n.): also letterhead, "sheet of paper with a printed or engraved logo or address," 1868, short for letterheading (1867); from letter (n.1) + heading (n.) in the printing sense. So called because it was printed at the "head" of the sheet of paper.
letter-man (n.): of college athletes, 1913, from letter (n.1) in the sports sense + man (n.).
letter-opener (n.): 1864 as a device to slit open letter envelopes, from letter (n.1) + opener. Earlier as a government or other official on continental Europe in charge of opening and reading private mails of suspected persons and censoring them (1847).
letter-perfect (adj.): 1833, in reference to exact memorization, from letter (n.1) + perfect (adj.).
letter-press (adj.): in reference to matter printed from relief surfaces, 1840, from letter (n.1) "a type character" + press (n.). Earlier "text," as opposed to copper-plate illustration (1771).
letter-rack (n.): 1849, from letter (n.1) + rack (n.1).
lettered (adj.): "literate, learned in letters," c. 1300, from letter (n.1). Meaning "inscribed" is from 1660s, from letter (v.).
lettering (n.): 1640s, "act of writing;" 1811, "act of putting letters on something;" 1796, "the letters marked or written on something," verbal noun from letter (v.).
letters (n.): "the profession of authorship or literature," mid-13c., from plural of letter (n.); as in Latin, French. Man of letters attested from 1640s.
letting (n.): "action of allowing movement or passage of something," early 15c., verbal noun from let (v.). Archaic or legalese meaning "delay, hindrance" is late Old English, from let (n.).
lettuce (n.): garden herb extensively cultivated for use as a salad, late 13c., letuse, probably somehow from Old French laitues, plural of laitue "lettuce" (cognate with Spanish lechuga, Italian lattuga), from Latin lactuca "lettuce," from lac (genitive lactis) "milk" (see lacto-); so called for the milky juice of the plant. Old English had bprrpwed the Latin word as lactuce.
leu (n.): monetary unit of Romania, introduced 1867, literally "lion." Monetary names in the Balkans often translate as "lion" because Dutch gold coins stamped with lions circulated widely in the region in the 17c. and the word for "lion" came to be a word for "money" in some languages there.
leucocyte (n.): see leukocyte.
leukaemia (n.): alternative spelling of leukemia.
leukemia (n.): progressive blood disease characterized by abnormal accumulation of leucocytes, 1851, on model of German Leukämie (1848), coined by R. Virchow from Greek leukos "clear, white" (from PIE *leuk- "light, brightness;" see light (n.)) + haima "blood" (see -emia). Formerly also leucemia.
leukemic (adj.): also leukaemic, leucemic, 1852; see leukemia + -ic.
leuko-: before vowels leuk-, also sometimes in Latinized form leuco-/leuc-, word-forming element used from 19c. and meaning "white" (or, in medicine, "leukocyte"), from Greek leukos "clear, white," from PIE *leuko-, suffixed form of *leuk- "light, brightness" (see light (n.)).
leukocyte (n.): also leucocyte, "white blood cell, white or colorless corpuscle of the blood or lymph," 1860, via French leucocyte, from leuco-, a Latinized comb. form of Greek leukos "white, clear," from PIE *leuk- "light, brightness" (see light (n.)) + -cyte (see cyto-).
lev (n.): monetary unit of Bulgaria, introduced 1881, literally "lion" (compare leu).
Levant: "Mediterranean lands east of Italy," especially the coastal region and islands of Asia Minor, Syria, and Lebanon, late 15c., from Middle French levant "the Orient" (12c.), from present participle of lever "to rise" (from Latin levare "to raise;" see lever). So called because the region was (from Western Europe) in the direction of sunrise. Related: Levanter.
Levantine (adj.): "of or pertaining to the Levant," 1640s, from Levant + -ine (1).
levari facias: old type of writ of execution against goods and profits of a debtor, legal Latin, literally "cause to be levied;" passive of levare "to raise" (see lever) + second person singular present subjunctive of facere "to make, do" (see factitious)