Old English fægen, fagen "glad, cheerful, happy, joyful, rejoicing," from a common Germanic root (cognates: Old Saxon fagan, Old Norse feginn "glad," Old High German faginon, Gothic faginon "to rejoice"), perhaps from PIE *pek- (1) "to make pretty." Often it means "glad" in a relative sense, "content to accept when something better is unobtainable." As an adverb, from c. 1200. Related: Fainly. Compare fawn (v.).
Middle English faunen, from Old English fagnian "rejoice, be glad, exult, applaud," from fægen "glad" (see fain); used in Middle English to refer to expressions of delight, especially a dog wagging its tail (early 14c.), hence "court favor, grovel, act slavishly" (early 15c.). Related: Fawned; fawning.
From late 14c. as "simulate (an action, an emotion, etc.)." Related: Feigned; feigning. The older spelling is that of faint, feint, but this word acquired a -g- in imitation of the French present participle stem feign- and the Latin verb.