Funnily enough, whilst "taffer" itself isn't quite a real word, "taff" is used as an informal word in Bulgarian, and means "pilfer" or "filch". One source comes from a translated version of "A Crime Wave at Blandings", published in 1997; the first Thief game, that "taffer" is attributed to, came out at 1998. Make of that what you will.
The following examples are all directly sourced from novels published in Bulgarian, translated back into English.
"... You've taffed my girl!" from "Feet of Clay", P.G. Wodehouse, 1997 translation
"That's why I taffed a car..." from "A Portrait in Death", Nora Roberts, 2003 translation
"... He'd try and taff it." from "Papillon", Henri CharriΓΒ¨re, 2010 translation
We can extrapolate that a taffer, back in English, is one who "taffs".
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