Affirm

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Middle English (in the sense ‘make firm’): via Old French from Latin affirmare, from ad- ‘to’ + firmus ‘strong’.

wiktionary

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From Middle English affermen, from Old French afermer, affermer, from Latin affirmare, adfirmare(“to present as fixed, aver, affirm”), from ad(“to”) + firmare(“to make firm”), from firmus(“firm”).

etymonline

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affirm (v.)

Middle English affermen, affirmen, "to decide upon" (c. 1300); "to state positively" (late 14c.), from Old French afermer (Modern French affirmer) "affirm, confirm; strengthen, consolidate," from Latin affirmare "to make steady, strengthen," figuratively "confirm, corroborate," from ad "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (from suffixed form of PIE root *dher- "to hold firmly, support").

The spelling was refashioned 16c. in French and English on Latin model. Legal sense "declare solemnly (as before a court) but without an oath" is from early 15c. Related: Affirmed; affirming.