"of or imparting filth," early 15c. metathesis of dritty "feculent; muddy" (late 14c.), from dirt + -y (2). Sense of characterized by dirt, unclean" is from 16c. Meaning "smutty, morally unclean" is from 1590s. Of colors, from 1690s. Sense of "not streamlined; rough, untidy, or imperfect" is by 1925. Of atomic bombs, "producing much radioactive fallout," by 1956.
Dirty linen "personal or familial secrets" is first recorded 1860s. Dirty work in the figurative sense is from 1764; dirty trick is from 1670s. Dirty joke is by 1856. The dirty look someone gives you is by 1923; dirty old man "superannuated lecher" is from 1932. Related: dirtiness.
of the skin or complexion, "of a sickly color, discolored, yellowish," Middle English salu, from Old English salo "dusky, dark" (related to sol "dark, dirty"), from Proto-Germanic *salwa- (source also of Middle Dutch salu "discolored, dirty," Old High German salo "dirty gray," Old Norse sölr "dirty yellow"), from PIE root *sal- (2) "dirty, gray" (source also of Old Church Slavonic slavojocije "grayish-blue color," Russian solovoj "cream-colored"). Related: Sallowness.
In early use often hardly more emphatic than the mod. dirty; it is now a violent expression of disgust, seldom employed in polite colloquial speech. [OED]
Related: Filthily; filthiness.