c. 1200, "false, treacherous, deceptive, deceitful, crafty" (obsolete), probably from Old English ficol "deceitful, cunning, tricky," related to befician "deceive," and to facen "deceit, treachery; blemish, fault." Common Germanic (compare Old Saxon fekan "deceit," Old High German feihhan "deceit, fraud, treachery"), from the same source as foe.
Sense of "changeable, inconstant, unstable" is from c. 1300 (especially of Fortune and women). Related: Fickleness. Fickly (c. 1300) is rare or obsolete. Also with a verb form in Middle English, fikelen "to deceive, flatter," later "to puzzle, perplex," which survived long enough in Northern dialects to get into Scott's novels. Fikel-tonge (late 14c.) was an allegorical or character name for "one who speaks falsehoods."