To put a cap or cover on something; to top or crown.
From 'be-' (causing or putting) plus 'cap' (from Old Norse 'káppr' or Latin 'cappa'). The 'be-' prefix made the noun into a transitive verb.
The 'be-' prefix could turn almost any noun into a verb in Middle English—you could 'becap,' 'becloak,' 'bering,' and hundreds of others, showing how flexible English word-formation was before standardization.
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