adult (adj.)

1530s (but not common until mid-17c.) "grown, mature," from Latin adultus "grown up, mature, adult, ripe," past participle of adolescere "grow up, come to maturity, ripen," from ad "to" (see ad-) + alescere "be nourished," hence, "increase, grow up," inchoative of alere "to nourish," from a suffixed form of PIE root *al- (2) "to grow, nourish."

Meaning "mature in attitude or outlook" is from 1929. As a euphemism for "pornographic," it dates to 1958 and does no honor to the word. In the old British film-rating system, A indicated "suitable for exhibit to adult audiences," and thus, implicitly, unsuitable for children (1914).

adult (n.)

"adult person," 1650s, from adult (adj.).

Others are reading

Definitions of adult from WordNet
adult (n.)
a fully developed person from maturity onward;
Synonyms: grownup
adult (n.)
any mature animal;
adult (adj.)
(of animals) fully developed;
an adult animal
Synonyms: big / full-grown / fully grown / grown / grownup
adult (adj.)
designed to arouse lust;
adult movies
Synonyms: pornographic