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1715, from French effronterie, from effronté "shameless," from Old French esfronte "shameless, brazen," probably from Late Latin effrontem (nominative effrons) "barefaced," from Latin ex- "out" (see ex-) + frontem (nominative frons) "brow" (see front (n.)).
Latin frontus had a sense of "ability to blush," but the literal sense of effrontery often has been taken to be "putting forth the forehead." Forehead in Johnson's Dictionary (1755) has a secondary sense of "impudence; confidence; assurance; audaciousness; audacity."