late 14c., "mental state," from Latin affectus "disposition, mood, state of mind or body produced by some external influence," noun use of adjective affectus "disposed, constituted, inclined," literally "furnished, supplied, endowed," past participle of afficere "to do; treat, use, manage, handle; act on, do something to; attack with disease; have influence on, apply force to," a verb used of many different actions, literally "to do to," from ad "to" (see ad-) + facere (past participle factus) "to make, do" (from PIE root *dhe- "to set, put"). Perhaps obsolete outside of psychology, where it is a modern coinage, translating German Affekt. Related: Affects.
"to make a pretense of," 1660s, earlier "to assume the character of (someone)," 1590s; originally in English in a now-obsolete sense "aim at, aspire to, desire" (early 15c.), from Old French afecter (15c.), later affecter, from Latin affectare "to strive after, aim at, aspire to," frequentative of afficere (past participle affectus) "to do something to, act on, influence" (see affect (n.)). Related: Affected; affecting.
"to make a mental impression on," 1630s; earlier "to attack" (c. 1600), "act upon, infect" (early 15c.), from affect (n.) or from Latin affectus "disposition, mood, state of mind or body produced by some external influence." Related: Affected; affecting. "The two verbs, with their derivatives, run into each other, and cannot be completely separated" [Century Dictionary].