afford (v.) Look up afford at
Middle English aforthen, from Old English geforðian "to put forth, contribute; further, advance; carry out, accomplish," from ge- completive prefix (see a- (1)) + forðian "to further," from forð "forward, onward" (see forth).

Change of -th- to -d- took place late 16c. (and also transformed burthen and murther into their modern forms). Prefix shift to af- took place 16c. under mistaken belief that it was a Latin word in ad-. Notion of "accomplish" (late Old English) gradually became "be able to bear the expense of, have enough money" to do something (late 14c.). Older sense is preserved in afford (one) an opportunity. Related: Afforded; affording.