early 15c., penetracioun, "a puncture, a penetrating wound," from Latin penetrationem (nominative penetratio) "a penetrating or piercing," noun of action from past-participle stem of penetrare "to put or get into, enter into" (see penetrate). From c. 1600 as "insight, discernment, shrewdness;" the sexual sense is attested from 1610s; meaning "act of penetrating or piercing" is from 1620s; in optics, by 1799.