adversary n.

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 French refashioned its written forms on the Latin model 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.

mid-15c., in legal case names, denoting action of one party against another, from Latin versus "turned toward or against," past participle of vertere "to turn," from PIE *wert- "to turn, wind," from root *wer- (2) "to turn, bend."

"involving adversaries," by 1892, from adversary (n.) + -al (1). The older adjective was simply adversary (late 14c.), but the tendency to confuse it in writing with the noun of the same form probably led to the creation of fresh adjectives (earlier examples are adversative, 1530s; adversarious, 1826). Related: Adversarially.