[ety. unknown; despite chronology OED suggests abbr. foil-cloy n., thence file-cloy under file v.1 . Weekley, Etymological Dict. of Modern English (1921), offers link to Fr. filou, a pickpocket; DSUE suggests SE file, a metal tool used to cut through things, and file, a rascal; 18C Fr. argot also has filer doux, to flatter, wheedle, ‘play the sleeping dog’, i.e. lie in wait](UK Und.)
[mid-17C–19C] a pickpocket.
[mid-18C] a shoplifter.
[early–mid-19C] (alsoold file upon the town) an experienced fraudster or confidence trickster.
[early 19C–1900s] an artful, cunning or shrewd person, a man, a ‘fellow’; thus old file, an old and/or experienced person.