Old English life (dative lif) "existence, lifetime, way of life, condition of being a living thing, opposite of death," from Proto-Germanic *libam (cf. Old Norse lif "life, body," Dutch lijf "body," Old High German lib "life," German Leib "body"), properly "continuance, perseverance," from PIE *leip- "to remain, persevere, continue; stick, adhere" (see leave (v.)). Much of the modern range of meanings was present in Old English. Meaning "property which distinguishes living from non-living matter" is from 1560s. Sense of "vitality, energy" is from 1580s. Extended 1703 to "term of duration (of inanimate objects)."
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.