Толковый словарь английского языка Oxford English Reference - ad-

prefix (also a- before sc, sp, st, ac- before c, k, q, af- before f, ag- before g, al- before l, an- before n, ap- before p, ar- before r, as- before s, at- before t) 1 with the sense of motion or direction to, reduction or change into, addition, adherence, increase, or intensification. 2 formed by assimilation of other prefixes (accurse; admiral; advance; affray). Etymology: (sense 1) (through OF a-) f. L ad to: (sense 2) a- repr. various prefixes other than ad-