1640s, "developed or ripe before the usual time," originally of plants, with -ous + Latin praecox (genitive praecocis) "maturing early," from prae "before" (see pre-) + coquere "to ripen," literally "to cook" (from PIE root *pekw- "to cook, ripen").
Originally of flowers or fruits. Figurative use, of persons, dates, etc., "characteristic of early maturity," by 1670s. Related: Precociously; precociousness. Obsolete princock "pert, forward, saucy boy or youth" (16c.-18c.) might be a rude, low slang folk-etymology alteration of Latin praecox.