Archaic or dialectal past participle or plural form; possibly related to crupper, a strap on a horse's saddle.
Likely a variant or inflected form of crupper, from Old French croupe (rump). The -en ending may represent an Old or Middle English plural or past participle form.
Cruppen shows how Old English used -en for plurals and past participles ('oxen,' 'beaten')—a grammatical pattern that faded away but left fossil traces in words like this!
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