late 15c., "to discolor, to make dirty," of uncertain origin, perhaps from Old French esmorcher "to torture," perhaps also "befoul, stain," from es- "out" (see ex-) + morcher "to bite," from Latin morsus, past participle of mordere "to bite" (see mordant). Sense perhaps influenced by smear. Sense of "dishonor, disgrace, discredit" first attested 1820.

1680s, "a soiling mark or smear," from smirch, v.. Figurative use by 1862.