early 14c., "offend by open disrespect," a figurative use, from Old French afronter "to face, confront; to slap in the face" (13c., Modern French affronter), from Late Latin affrontare "to strike against," from Latin ad frontem "to the face," from ad "to" (see ad-) + frons (genitive frontis) "forehead, front" (see front (n.)). Related: Affronted; affronting.
1590s, "an openly offensive word or deed," from affront (v.) or from French affront (n.), from the verb in French.
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word-forming element expressing direction toward or in addition to, from Latin ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE root *ad- "to, near, at."
Simplified to a- before sc-, sp- and st-; modified to ac- before many consonants and then re-spelled af-, ag-, al-, etc., in conformity with the following consonant (as in affection, aggression). Also compare ap- (1).
In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but written forms in French were refashioned after Latin in 14c. and English did likewise 15c. in words it had picked up from Old French. In many cases pronunciation followed the shift. Over-correction at the end of the Middle Ages in French and then English "restored" the -d- or a doubled consonant to some words that never had it (accursed, afford). The process went further in England than in France, where the vernacular sometimes resisted the pedantic, resulting in English adjourn, advance, address, advertisement (Modern French ajourner, avancer, adresser, avertissement). In modern word-formation sometimes ad- and ab- are regarded as opposites, but this was not in classical Latin.
Sense "foremost part of anything" emerged in the English word mid-14c.; sense of "the face as expressive of temper or character" is from late 14c. (hence frontless "shameless," c. 1600). The military sense of "foremost part of an army" (mid-14c.) led to the meaning "field of operations in contact with the enemy" (1660s); home front is from 1919. Meaning "organized body of political forces" is from 1926. Sense of "public facade" is from 1891; that of "something serving as a cover for illegal activities" is from 1905. Adverbial phrase in front is from 1610s. Meteorological sense first recorded 1921.
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.).