affordyoudaoicibaDictYouDict[afford 词源字典]
afford: [OE] This verb originally meant ‘accomplish, fulfil’. In Old English times it was geforthian, formed from the prefix ge-, denoting completion of an action, and forthian ‘advance towards completion’ or literally ‘further’ (from the adverb forth). The notion of accomplishing something or managing something gradually led, by the 15th century, to the idea of being able to do something because one has enough money.

Meanwhile, the original ge- prefix, which by Middle English times had become i- (iforthien), had been transformed into af- under the influence of the many Latin-based words beginning in aff-, and in the 16th century spellings with final d in place of th start to appear.
=> forth[afford etymology, afford origin, 英语词源]
afford (v.)youdaoicibaDictYouDict
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.