also half life, 1864, "unsatisfactory way of living," from half + life; the sense in physics, "amount of time it takes half a given amount of radioactivity to decay" is first attested 1907.
Entries linking to half-life
half
Old English half, halb (Mercian), healf (W. Saxon) "side, part," not necessarily of equal division (original sense preserved in behalf), from Proto-Germanic *halba- "something divided" (source also of Old Saxon halba, Old Norse halfr, Old Frisian, Middle Dutch half, German halb, Gothic halbs "half"), a word of no certain etymology. Perhaps from PIE root *skel- (1) "to cut," or perhaps a substratum word. Noun, adjective, and adverb all were in Old English.
Used also in Old English phrases, as in modern German, to mean "one half unit less than," for example þridda healf "two and a half," literally "half third." The construction in two and a half, etc., is first recorded c. 1200. Of time, in half past ten, etc., first attested 1750; in Scottish, the half often is prefixed to the following hour (as in German, halb elf = "ten thirty").
To go off half-cocked in the figurative sense "speak or act too hastily" (1833) is in allusion to firearms going off prematurely; half-cocked in a literal sense "with the cock lifted to the first catch, at which position the trigger does not act" is recorded by 1750. In 1770 it was noted as a synonym for "drunk." Bartlett ("Dictionary of Americanisms," 1848) writes that it was "a metaphorical expression borrowed from the language of sportsmen, and is applied to a person who attempts a thing in a hurry without due preparation, and consequently fails."
life n.
Old English life (dative lif) "animated corporeal existence; lifetime, period between birth and death; the history of an individual from birth to death, written account of a person's life; way of life (good or bad); condition of being a living thing, opposite of death; spiritual existence imparted by God, through Christ, to the believer," from Proto-Germanic *leiban (source also of Old Norse lif "life, body," Old Frisian, Old Saxon lif "life, person, body," Dutch lijf "body," Old High German lib "life," German Leib "body"), properly "continuance, perseverance," from PIE root *leip- "to stick, adhere."
The noun associated with live (v.) "to live," which is literally "to continue, remain." Extended 1703 to inanimate objects, "term of duration or existence." Sense of "vitality, energy in action, expression, etc." is from 1580s. Meaning "conspicuously active part of human existence, pleasures or pursuits of the world or society" is by 1770s. Meaning "cause or source of living" led to the sense "vivifying or animating principle," and thus "one who keeps things lively" in life of the party (1787). Meaning "imprisonment for life, a life sentence" is from 1903. Paired alliteratively with limb from 1640s. Not on your life "by no means" is attested from 1896.
In gaming, an additional turn at play for a character; this transferred use was prefigured by uses in card-playing (1806), billiards (1856), etc., in reference to a certain number of chances or required objects without which one's turn at the game fails. The life "the living form or model, semblance" is from 1590s. Life-and-death "of dire importance" is from 1822; life-or-death (adj.) is from 1897. Life-jacket is from 1840; life-preserver from 1630s of anything that is meant to save a life, 1803 of devices worn to prevent drowning. Life-saver is from 1883, figurative use from 1909, as a brand of hard sugar candy from 1912, so called for shape.
Life-form is from 1861; life-cycle is from 1855; life-expectancy from 1847; life-history in biology from 1870; life-science from 1935. Life-work "the labor to which one's life has been devoted" is from 1848. Expression this is the life is from 1919; verbal shrug that's life is from 1924 (earlier such is life, 1778).