premature (adj.)

mid-15c., "ripe;" 1520s, "existing or done before the proper or usual time, arriving too early at maturity," from Latin praematurus "early ripe" (as fruit), "too early, untimely," from prae "before" (see pre-) + maturus "ripe, timely" (see mature (v.)). Related: Prematurely; prematurity; prematuration.

Premature ejaculation is attested from 1848; the Latin euphemism ejaculatio praecox dates to 1891 in English but was used earlier in German and appears to have been, at first at least, the psychologist's term for it.