assimilated form of L. ad- ‘to,’ bef. initial p- as L. ad-proba-, ap-proba-. In OFr. this ad- ap- was, by regular phonetic law, reduced to a- (as in the separate word ad), and in this form the Fr. words were adopted in Eng. as a-part, a-pere, a-ply, a-pose, a-prise, a-prove. In imitation of the Latin forms, the scribes began to double the p in Fr. spelling in 14th c., and in Eng. in 15th, though, in speech, the prefix is still really a-. By mistake ap- was also substituted for a- in several words where it had a different origin, as appair, appeach: see AD- pref. 2, A- pref. 10.
