"shamelessness, impudence, boldness in transgressing the bounds of modesty and propriety," 1715, from French effronterie, from effronté "shameless," from Old French esfronte "shameless, brazen," probably from Late Latin effrontem (nominative effrons) "barefaced, shameless," from assimilated form of Latin ex "out" (see ex-) + frontem (nominative frons) "brow" (see front (n.)). Also compare affront.
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." English had an earlier verb effront "treat with effrontery" (17c.).