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lethargy: lethargy: [14] Greek léthē meant ‘oblivion’ (the Romans used it for the name of a river in Hades whose water induced forgetfulness, and its influence has also been traced in changing Latin lētum ‘death’ to lēthum, source of English lethal [17]). From it was formed the adjective léthargos, which in turn produced the noun lēthargíā, source (via Latin and Old French) of English lethargy.
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plethora: plethora: [16] Greek plēthórē meant ‘fullness’ (it was derived from the verb pléthein ‘fill’, a descendant of the Indo-European base *plē-, from which English gets full, plenty, etc). It was taken over into late Latin as plēthōra, and at first was widely used as a medical term, denoting an ‘excess of blood or other fluids in the body’. That was what it originally denoted in English, but by the end of the 16th century the more general ‘surplus’ was coming into use.
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shibboleth: shibboleth: [14] Hebrew shibbōleth meant ‘stream’. According to the Bible, the Gileadites used it as a password, for they knew their enemies the Ephraimites could not pronounce the sh properly (‘And it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay, then they said unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right’, Judges 12:5–6).

In 17th-century English it came to be applied generically to any word used as a test of pronunciation, particularly as a sign of belonging to a group, and hence by extension to any catchword or slogan adopted by a group, and this eventually evolved into the modern sense ‘outmoded slogan, practice, etc still adhered to’.
Alethea: fem. proper name, from Greek aletheia "truth, truthfulness," from alethes "true," literally "not concealing," from privative prefix a- "not" (see a- (3)) + lethe "forgetfulness, oblivion" (see latent).
doublethink (n.): 1948, coined by Orwell in "Nineteen Eighty-Four," from double (adj.) + think.
lethal (adj.): 1580s, from Late Latin lethalis, alteration of Latin letalis "deadly, fatal," from letum "death," of uncertain origin. Form altered in Late Latin by association with lethes hydor "water of oblivion" in Hades in Greek mythology, from Greek lethe "forgetfulness."
lethality (n.): 1650s, from lethal + -ity.
lethargic (adj.): late 14c., litargik, from Latin lethargicus "affected with lethargy," from Greek lethargikos, from lethargos (see lethargy). Related: Lethargically.
lethargy (n.): late 14c., litarge, from Old French litargie or directly from Medieval Latin litargia, from Late Latin lethargia, from Greek lethargia "forgetfulness," from lethargos "forgetful," originally "inactive through forgetfulness," from lethe "forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English.
Lethe: river of Hades (whose water when drunk caused forgetfulness of the past), from Greek lethe, literally "forgetfulness, oblivion," related to lethargos "forgetful," lathre "secretly, by stealth," lathrios "stealthy," lanthanein "to be hidden." Cognate with Latin latere "to be hidden" (see latent). Related: Lethean.
plethora (n.): 1540s, a medical word for "excess of body fluid," from Late Latin plethora, from Greek plethore "fullness," from plethein "be full" (see pleio-). Figurative meaning "too-muchness, overfullness in any respect" is first recorded 1700. Related: Plethoric.
shibboleth (n.): late 14c., the Hebrew word shibboleth, meaning "flood, stream," also "ear of corn;" in Judges xii:4-6. It was the password used by the Gileadites to distinguish their own men from fleeing Ephraimites, because Ephraimites could not pronounce the -sh- sound. Hence the figurative sense of "watchword" (first recorded 1630s), which evolved by 1862 to "outmoded slogan still adhered to." A similar test-word was cicera "chick pease," used by the Italians to identify the French (who could not pronounce it correctly) during the massacre called the Sicilian Vespers (1282).
telethon (n.): prolonged TV fundraiser, 1949, from television + marathon (see -athon). Milton Berle's 16-hour television cancer fundraiser in April 1949 might have been the first to be so called.
alethic: "Denoting modalities of truth, such as necessity, contingency, or impossibility", 1950s: from Greek alētheia 'truth' + -ic.
alethiology: "The study of truth; that part of logic or philosophy which deals with the nature of truth", Mid 19th cent.; earliest use found in Henry Longueville Mansel (1820–1871), dean of St Paul's and theologian. From post-classical Latin alethiologia (1764, after German Alethiologie ( J. H. Lambert Neues Organon) from post-classical Latin alethia (late 2nd or early 3rd cent. in Tertullian) or its etymon ancient Greek ἀλήθεια truth (from ἀληθής true + -εία) + post-classical Latin -ologia.
isopleth: "A line on a map connecting points having equal incidence of a specified meteorological feature", Early 20th century: from Greek isoplēthēs 'equal in quantity', from Greek isos 'equal' + plēthos 'multitude, quantity'.
median lethal dose: "The dose of a drug or of ionizing radiation that kills 50 per cent of test subjects; symbol LD50", 1920s.