- ad- prefix expressing direction toward or in addition to, from L. ad "to, toward" in space or time; "with regard to, in relation to," as a prefix, sometimes merely emphatic, from PIE *ad- "to, near, at" (cognate with O.E. æt; see AT (Cf. 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 (e.g. affection, aggression). In Old French, reduced to a- in all cases (an evolution already underway in Merovingian Latin), but written forms were refashioned after Latin in 14c. in French and 15c. in English words picked up from Old French. In many cases pronunciation followed the shift.
Etymology dictionary. 2014.