defile v.

active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.

As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.

c. 1300, foilen "to spoil a trace or scent by running over it" (more commonly defoilen), irregularly from Old French foler, fuler "trample on, injure, maim; ill-treat, deceive, get the better of" (13c., Modern French fouler), from Vulgar Latin *fullare "to clean cloth" (by treading on it), from Latin fullo "one who cleans cloth, a fuller," which is of unknown origin. Compare full (v.).

Hence, "to overthrow, defeat" (1540s; as a noun in this sense from late 15c.); "frustrate the efforts of" (1560s). Related: Foiled; foiling. Foiled again! as a cry of defeat and dismay is from at least 1847.