To thumb or handle something roughly with the thumbs; to soil or mark by touching with thumbs.
From the prefix 'be-' combined with 'thumb,' forming a verb in the Middle English pattern. This follows the same verb-creation system where 'be-' makes an action verb from a body part or object.
Medieval English was obsessed with creating vivid verbs for specific actions—'bethumb' is delightfully specific about the kind of damage caused by careless thumbing. It's why 'befriend' still survives but 'bethumb' didn't—some 'be-' verbs were just too perfectly ordinary to last.
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