incest: [13] Etymologically, incest is virtually the same word as unchaste. It was borrowed from Latin incestus, a noun use of an adjective formed from the negative prefix in- and castus ‘pure’ (source of English chaste). The Latin word denoted ‘unchastity’ in general, but in practice was often applied specifically to ‘sexual contact between close relatives’. => chaste[incest etymology, incest origin, 英语词源]