In a list of words you wouldn’t believe were real unless we provided the evidence, this one would come pretty close to the top. It popped up on HH today: to fillip is to fling a hedgehog into the air using a tilted plank of wood.
The key to this entry is that little number 5 ahead of that entry from the English Dialect Dictionary: this is just one of a number of different definitions of the word fillip on record, and, mercifully, it’s the most unusual.
In its more general, less animal-cruelty-orientated sense, the word fillip means simply “to propel something with effort”; when it first appeared in the language in the sixteenth century, it meant merely to flip or flick something, or to toss a coin. The hedgehog-tossing version is a much later development, apparently one of a handful of bizarrely cruel pranks that emerged in the nineteenth century (remember spanghewing?).
Etymologically, there’s little to report. Fillip—like flick, flip and flirt—is probably nothing more than an onomatopoeia, meant somehow to echo the sound or the action of filliping something.
Although filliping a hedgehog probably sounds a little more unusual than that.