smut (n.) Look up smut at Dictionary.com
1660s, "black mark, stain," from verb smutten "debase, defile" (late 14c.), later "stain or mark with soot, etc." (1580s), cognate with Middle High German smotzen "make dirty," from West Germanic *smutt- (cognates: Middle High German smuz "grease, dirt;" German Schmutz "dirt," schmutzen "to make dirty"). The meaning "indecent or obscene language" is first attested 1660s.