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- (source also of Middle High German smuz "grease, dirt;" German Schmutz "dirt," schmutzen "to make dirty"). The meaning "indecent or obscene language" is first attested 1660s.
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