To make someone cautious or suspicious; to put on guard.
From Middle English, combining the prefix 'be-' (meaning to make or cause) with 'wary' (from Old English 'wær' meaning cautious). The prefix 'be-' was commonly used in Old and Middle English to create verbs from adjectives.
The prefix 'be-' was incredibly productive in Middle English, creating hundreds of verbs that have mostly disappeared—bewary is one of the rare survivors. Linguists call this kind of creative word-formation 'derivation,' and it shows how English speakers could transform almost any adjective into an action verb.
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