defile
From Late Middle English defilen (“to make dirty, befoul; rape; abuse; destroy; injure; oppress”) [and other forms], a variant of defoulen (“to make dirty, defile, pollute; have sexual intercourse with; rape; etc.”) (compare also defoilen). Defoulen is a blend of Middle English foulen (“to make dirty, soil, pollute”) (from the adjective foul (“dirty, rotten, stinking, corrupt, sinful, guilty”) and Old English fūlian (“to decay”)), and Old French defoler, defouler (“to trample, crush; destroy”), from de- (intensifying prefix) + foler, fouler, fuller (“to trample, tread on; mistreat, oppress, destroy”) (from Vulgar Latin fullāre (“to full (make cloth denser and firmer by soaking, beating, and pressing)”), from Latin fullō (“person who fulls cloth, fuller”); further etymology uncertain, possibly from Proto-Indo-European *bʰleh₃- (“to blow; to inflate, swell; to bloom, flower”) or Etruscan 𐌘𐌖𐌋𐌖 (φulu)). The English word is analysable as de- + file (“to corrupt; defile”).
The Middle English word defilen was probably formed from defoulen on the analogy of befilen (“to make dirty, befoul; corrupt; violate one's chastity; desecrate; slander”) and befoulen (“to make dirty, befoul; violate one's chastity; vilify”), respectively from Old English befȳlan (“to befoul, pollute, defile, make filthy”) (compare also Middle English filen (“to make foul, impure, or unclean, pollute; pollute morally or spiritually; desecrate, profane; have sexual intercourse with; rape; etc.”)) and foulen (“to make dirty, pollute; become dirty; defecate; deface or deform; pollute morally or spiritually; damage, injure; destroy; treat unfairly, oppress; tread on, trample”). Filen and foulen are respectively from Old English fȳlan (“to befoul, defile, pollute”) and Old English fūlian (“to foul”), from Proto-West Germanic *fūlijan (“to make dirty, befoul”) and *fūlēn (“to become foul, decay”), both ultimately from Proto-Germanic *fūlaz (“dirty, foul; rotten”), from Proto-Indo-European *puH- (“foul; rotten”). See foul.
Cognates
* German Low German befulen (“to defile, sully”)
* Dutch bevuilen (“to defile, soil”)
* Scots befile (“to befoul, dirty”)
* West Frisian befûjle (“to soil”)
PIE word
 *dwís
The verb is borrowed from French défiler (“to march; to parade”), from dé- (prefix indicating actions are done more strongly or vigorously) + one or both of the following:
* filer (“to thread through (a crowd)”) (from Late Latin filāre, from Latin fīlum (“fibre, filament, string, thread”), ultimately from Proto-Indo-European *gʷʰiH-(s-)lo-).
* file (“line of objects placed one after the other, file”), from filer (see above), or fil (“thread, yarn; wire”), from Old French fil, from Latin fīlum (see above).
The noun is borrowed from French défilé (“parade, procession”), a noun use of the past participle of défiler (verb); see above.
The verb is borrowed from French défiler (“to arrange soldiers or fortify (something) as a protection from enfilading fire; to unthread”) (compare Middle French desfilher (“to unthread”)), from dé- (prefix meaning ‘not’) + enfiler (“to rake with gunfire, enfilade; to string on to a thread; to thread (a needle)”) (from en- (prefix meaning ‘in, into; on, on to’) + filer (verb) or file (noun); see etymology 2).
The noun is derived from the verb.