My first acquaintance with this word was in the phrase filch a pilchard (though I’ll be damned if I can remember where I first saw it; when I Google the phrase, the first hit to use it is Sesquiotica). I’m not sure why one would incline to pilfering a small fish; one could as easily filch a flitch (a side of bacon) or a finch – although the finch might flinch, if it were quick.
Whatever you filch, though, it’s a peccadillo. You don’t filch a car or a million dollars – not unless you want to humorously play the theft as a minor matter (sort of like calling a mansion a “pile of bricks”). Filching is a dirty little thing, not a dirty big thing, and it’s quick and surreptitious (as Merriam-Webster reminds us). It has a sound perhaps of a hand grasping dry dust, like in a gulch, or something ground up, like mulch; it could be nothing more than rude air, like belch, or its constriction and stoppage, like squelch. But one thing’s clear: it’s closer to zilch than to much.
English filches words by the bagful, as we know. But did it filch filch? Probably not, but no one is absolutely certain. The Oxford English Dictionary, somewhat behind the curve on this, says “Of unknown origin.” Others are more venturesome: perhaps it comes from German filzen ‘comb through’; perhaps, as Dictionary.com says, it starts with “Middle English filchen to attack (in a body), take as booty, Old English fylcian to marshal (troops), draw (soldiers) up in battle array, derivative of gefylce band of men; akin to folk.” Our language is so fickle, we sometimes end up filling in the blanks with whatever we can grab. And afterwards, we swear that it just fell off the truck.