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.