Etymology
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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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legionnaire (n.)
1818, from French légionnaire, from légion (see legion). Legionnaires' Disease, caused by Legionella pneumophilia, was named after the lethal outbreak of July 1976 at the American Legion convention in Philadelphia's Bellevue Stratford Hotel. Hence also Legionella as the name of the bacterium.
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neutron (n.)

"electrically neuter particle of the atom," 1921, coined by U.S. chemist William D. Harkins (1873-1951) from neutral (adj.) + -on. First record of neutron bomb, which releases a large number of lethal neutrons but produces little blast, is from 1960. Neutron star attested from 1934, originally hypothetical; so called because it would be composed of densely packed neutrons.

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cholera (n.)

late 14c., "bile, melancholy" (originally the same as choler), from French cholera or directly from Late Latin cholera, from Greek kholera "a type of disease characterized by diarrhea, supposedly caused by bile" (Celsus), from khole "gall, bile," so called for its color, related to khloazein "to be green," khlōros "pale green, greenish-yellow," from PIE root *ghel- (2) "to shine," with derivatives denoting "green, yellow," and thus "bile, gall." But another sense of khole was "drainpipe, gutter."

Revived 1560s in classical sense as a name for a severe digestive disorder (rarely fatal to adults); and 1704 (especially as cholera morbus), for a highly lethal disease endemic in India, periodically breaking out in global epidemics, especially that reaching Britain and America in the early 1830s.

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booby (n.)

"stupid person," 1590s, from Spanish bobo "stupid person," also used of various ungainly seabirds, probably from Latin balbus "stammering," from an imitative root (see barbarian).

Specific sense "dunce in a school class" is by 1825. Booby prize "object of little value given to the loser of a game," is by 1884:

At the end of every session the dominie had the satirical custom of presenting his tawse [a corporal punishment implement used for educational discipline] as a "booby-prize" to some idle or stupid lout whom he picked out as meriting this distinction so that next time they met he might start fresh and fair with new pair for a new set of classes. [Ascott R. Hope, "Dumps," Young England magazine, Sept. 1884]

Booby trap is by 1850, originally a schoolboy prank; the more lethal sense developed during World War I. Booby-hatch "wooden framework used to cover the after-hatch on merchant vessels" is from 1840; as "insane asylum" by 1936.

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