effronteryyoudaoicibaDictYouDict[effrontery 词源字典]
effrontery: [18] The notion of ‘audacity’ or ‘impudence’ is often expressed in terms of ‘exposing or pushing forward the face’: a ‘barefaced lie’ or ‘putting on a bold front’, for instance. And effrontery is no exception. It comes ultimately from late Latin effrōns ‘barefaced, shameless’, a compound adjective formed from the prefix ex- ‘out of’ and frōns ‘forehead’ (source of English front).

This seems subsequently to have been reformulated along the lines of its original components, giving Vulgar Latin *exfrontātus, source of Old French esfronte. This in turn developed to French effronté, whose derived noun effronterie was acquired by English as effrontery.
=> front[effrontery etymology, effrontery origin, 英语词源]
effrontery (n.)youdaoicibaDictYouDict
"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 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.).