af-, prefix
1. Assimilated form of L. ad- to, bef. initial f-, as in af-fectus, af-flātus, af-fluentia. Reduced in OFr. to a-, and so adopted in Eng. But artificially restored in Fr. spelling in 4–5, whence extended to Eng. in 5–6; as in affair, affiance, affront, early Eng. afaire, afiaunce, afront. In all mod. words from L., aff- has been written from the first, though only one f- is pronounced.
2. While the words from OFr. were being refashioned after L. spelling, af- was substituted for a- in various words where it did not represent L. af-, as in a(f)fray, a(f)fright; see ad- 2.