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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 "manage to buy or maintain; have enough money (to do something)" (1833). Older sense is preserved in afford (one) an opportunity. Related: Afforded; affording.