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late 14c., "first in order," from Latin primus "first, the first, first part," figuratively "chief, principal; excellent, distinguished, noble" (source also of Italian and Spanish primo), from pre-Italic *prismos, superlative of PIE *preis- "before," from root *per- (1) "beyond, through" (see per).
Meaning "first in importance" is from 1610s in English; that of "first-rate" is from 1620s. Arithmetical sense (e.g. prime number) is from 1560s; prime meridian is from 1878. Prime time originally (c.1500) meant "spring time;" broadcasting sense of "peak tuning-in period" is attested from 1961.
"earliest canonical hour" (6 a.m.), Old English prim, from Medieval Latin prima "the first service," from Latin prima hora "the first hour" (of the Roman day). Meaning "most vigorous stage" first recorded 1530s; specifically "springtime of human life" (often meaning ages roughly 21 to 28) is from 1590s. In classical Latin, noun uses of the adjective meant "first part, beginning; leading place."
"to fill, charge, load" (a weapon), 1510s, probably from prime (adj.). Meaning "to cover with a first coat of paint or dye" is from c.1600. To prime a pump (c.1840) meant to pour water down the tube, which saturated the sucking mechanism and made it draw up water more readily. Related: Primed; priming.