Mid kent in my case. From wikipedia (so it *must* be true):
Fainites and fains (or vainites and vains) predominated in London and throughout southern England, apart from the scribs and screams of east Hampshire, and extended north as far as Olney in Buckinghamshire. Variations included fennits, fannies, fainsies, faylines, vainlights and vainyards. Notes and Queries reported in 1870 that fains was in common use by London schoolboys. Faints appeared in an 1889 dictionary of slang and fainits in 1891. According to philologist J. R. R. Tolkien, the term derives from the medieval term fein I, descended in turn from the Old French se feindre meaning "to make excuses, hang back or back out of battle". He also proposes that this use of the term throws light on line 529 of the Clerk's Tale by Chaucer that "lordes heestes mowe nat been yfeyned" (the lords orders cannot be treated with a fain I; in other words, declined).[2] Another translation of the Anglo-Norman word feindre is "pretend, feign, turn a blind eye to", which is what the more powerful child does whilst granting respite.
So to sum up, it seems no one really knows :-)
Darren