Origin and history of midlife

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," reconstructed in Watkins to be from PIE root *leip- "to stick, adhere."

The transition toward 'live', etc. is only Gmc. and disputed, but semantically comprehensible: stick → remain (also Tocharian) → live (Gmc.). [Boutkan]

The noun associated with live (v.) "to live," which is literally "to continue, remain."

The sense was extended by 1703 to inanimate objects, "term of duration or existence." The sense of "vitality, energy in action, expression, etc." is from 1580s. The meaning "conspicuously active part of human existence, pleasures or pursuits of the world or society" is attested by 1770s.

The meaning "cause or source of living" led to the sense of "vivifying or animating principle," and thus "one who keeps things lively" in life of the party (1787). The meaning "imprisonment for life, a life sentence" is by 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," it is a transferred use prefigured by similar 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.

THE Latin language and the English are alike poorer than the Greek, in having but one word, the Latin 'vita,' the English 'life,' to express these two Greek. There would, indeed, be no comparative poverty here, if [zōē] and [bios] were merely duplicates; but, covering as they do very different spaces of meaning, it is certain that we, having but one word for them both, must use this one in very diverse senses; it is possible that by this equivocation we may, without being aware of it, conceal very real and important differences from ourselves; for, indeed, there is nothing so potent to do this as the equivocal use of a word. [Archbishop Trench, "Synonyms of the New Testament," 1854]

Life-and-death "of dire importance" is from 1822; life-or-death (adj.) "critical, desperate" is attested from 1856.

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 attested by 1861. Life-work "the labor to which one's life has been devoted" is from 1848.

The expression this is the life is from 1919; the verbal shrug that's life is attested by 1884 (sometimes that's life in a nutshell; earlier such is life, 1778); earlier that's life! was an affirmation of vivid experience, "that's living! that's being alive!"

And when, at the last, he wuz lyin'

  At the end o' the toil an' the strife,

An' the preacher says: "Ol' man, you're dyin'."

  He whistled and said, "Well, that's life!"

[Frank L. Stanton, "A True Philosopher," 1898]

"middle; being the middle part or midst; being between, intermediate," Old English mid, midd from Proto-Germanic *medja- (source also of Old Norse miðr, Old Saxon middi, Old Frisian midde, Middle Dutch mydde, Old High German mitti, German mitte, Gothic midjis "mid, middle"), from PIE root *medhyo- "middle."

By late Middle English probably felt as a prefix only, and now surviving in English only as a prefix (mid-air, midstream, etc.). Prefixed to months, seasons, etc. from late Old English. As a preposition, "in the middle of, amid" (c. 1400) it is from in midde or a shortened form of amid (compare midshipman) and sometimes is written 'mid.