U.S. slang; said in "Dictionary of American Slang" to be originally 1920s army and 1930s college student slang for "venereal disease." Thus by 1940, "dirty, disreputable person," and by 1950, "undesirable impurity." By 1945 (with various modifiers) it was the G.I.'s name for disease of any and every sort."
Perhaps this word is a continuation of crud as the old metathesis variant of curd (q.v.), which would make it an unconscious return to the original Middle English form of that word. Century Dictionary (1897) has crud only in the sense "Obsolete or dialectal form of curd."
"coagulated or thickened part of milk," c. 1500, metathesis of crud (late 14c.), which originally was "any coagulated substance," probably from Old English crudan "to press, drive," perhaps via ancestor of Gaelic gruth (because cognates are unknown in other Germanic or Romance languages) from a PIE *greut- "to press, coagulate."
late 14c., "in a raw or unprepared state" (of coarse bread or untanned hide), from Latin crudus "rough; not cooked, raw, bloody," from PIE *krue-do-, from PIE root *kreue- "raw flesh." Meaning "lacking grace, socially unrefined" is attested by 1640s. Related: Crudely; crudeness. Crude oil, which is in its natural state and unrefined, is from 1865.
"mixed raw vegetables as a hors d'oeuvre," 1960, from French crudités, literally "raw things" (see crudity).
early 15c., of food, "quality of producing unnatural humors," from Old French crudité (14c.) and directly from Latin cruditatem (nominative cruditas) "indigestion," from crudus "rough; not cooked, raw, bloody" (see crude). From 1620s as "that which is crude;" 1630s as "quality or state of being crude."