To furnish with horns; to make horned or to mark with horns.
From 'be-' prefix plus 'horn.' Following the pattern of transformative 'be-' verbs, this would mean to provide or equip with horns, though the word is essentially obsolete.
The verb 'behorn' connects to an old meaning of 'horn' as a symbol of cuckoldry—calling someone 'horned' meant their spouse was unfaithful—so 'behorn' had an insulting, metaphorical edge.
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