"answering 'yes,' " mid-15c., from use in logic; from Old French affirmatif, earlier afirmatif (13c.), from Latin affirmativus, from affirmat-, past participle stem of affirmare "to make steady; strengthen; confirm," from ad "to" (see ad-) + firmare "strengthen, make firm," from firmus "strong" (see firm (adj.)).
As a noun from early 15c., "that which affirms or asserts." American English affirmative action "positive or corrective effort by employers to prevent discrimination in hiring or promotion" is attested from 1935 with regard to labor unions (reinstatement of fired members, etc.). The specific racial sense is attested from 1961; by late 1970s the sense had shifted toward pro-active methods such as hiring quotas. Related: Affirmatively.