Why is the ninth month called September?
c.1500, from dirt + -y (2). Earlier dritty (late 14c.). Meaning "smutty, morally unclean" is from 1590s. Of colors, from 1690s. Dirty linen "personal or familial secrets" is first recorded 1860s. Dirty work in the figurative sense is from 1764; dirty trick is from 1670s. The dirty look someone gives you is from 1928; dirty old man "superannuated lecher" is from 1932. Related: dirtiness.
1590s, from dirty (adj.). Related: Dirtied; dirtying.