To treat someone as a scoundrel; to call someone a scoundrel or act toward them scornfully.
From 'be-' + 'scoundrel' (from Italian 'scoundrel' or possibly French 'escondrire'). This transforms the noun 'scoundrel' (a dishonest person) into a verb meaning to condemn someone as such.
This is a delightfully rare word that shows how English writers would take any noun and verb-ify it with 'be-' to create colorful insults. Imagine calling someone a 'scoundrel' wasn't strong enough, so you'd 'bescoundrel' them instead!
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