Etymology
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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 lēthē, literally "forgetfulness, oblivion," from PIE root *ladh- "be hidden" (see latent). Related to lēthargos "forgetful" and cognate with Latin latere "to be hidden." Also the name of a personification of oblivion, a daughter of Eris. Related: Lethean.

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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).
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Alethea 

fem. proper name, from Greek aletheia "truth, truthfulness," from alethes "true," literally "not concealing," from privative prefix a- "not" (see a- (3)) + lēthē "forgetfulness, oblivion, concealment," from PIE root *ladh- "be hidden" (see latent).

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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 lēthargia "forgetfulness," from lēthargos "forgetful," apparently etymologically "inactive through forgetfulness," from lēthē "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.

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latent (adj.)

mid-15c., "concealed, secret," from Latin latentem (nominative latens) "lying hid, concealed, secret, unknown," present participle of latere "lie hidden, lurk, be concealed," from PIE *late-, suffixed form of root *lādh- "to be hidden" (source also of Greek lēthē "forgetfulness, oblivion," lēthargos "forgetful," lathre "secretly, by stealth," lathrios "stealthy," lanthanein "to be hidden;" Old Church Slavonic lajati "to lie in wait for"). Meaning "dormant, undeveloped" is from 1680s, originally in medicine.

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guillotine (n.)

"The name of the machine in which the axe descends in grooves from a considerable height so that the stroke is certain and the head instantly severed from the body." [Universal Magazine of Knowledge and Pleasure, January 1793], 1791, from French guillotine, named in recognition of French physician Joseph Guillotin (1738-1814), who as a deputy to the National Assembly (1789) proposed, for humanitarian and efficiency reasons, that capital punishment be carried out by beheading quickly and cleanly on a machine, which was built in 1791 and first used the next year. Similar devices on similar principles had been used in the Middle Ages. The verb is attested by 1794. Related: Guillotined; guillotining.

This is the product of Guillotin's endeavors, ... which product popular gratitude or levity christens by a feminine derivative name, as if it were his daughter: La Guillotine! ... Unfortunate Doctor! For two-and-twenty years he, unguillotined, shall hear nothing but guillotine, see nothing but guillotine; then dying, shall through long centuries wander, as it were, a disconsolate ghost, on the wrong side of Styx and Lethe; his name like to outlive Cæsar's. [Carlyle, "French Revolution"]
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