to cause someone to worry or fret excessively; to make anxious or troubled.
From Old English 'be-' prefix meaning 'around' or 'thoroughly' combined with 'fret' (to worry or wear away), dating to before 1200. The prefix intensifies the base verb's meaning.
The 'be-' prefix is a linguistic workhorse that shows up in hundreds of verbs like 'befuddle,' 'befriend,' and 'besmirch'—it's like an intensifier button that Old English speakers could attach to almost any verb to amplify its effect.
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