To worry or stress about something, or the ridged bars on a guitar neck that help you play different notes.
From Old English 'fretan' meaning 'to eat away' or 'to consume.' This evolved into meaning 'to wear away emotionally' (worrying), and separately became a musical term for the bars that divide the fingerboard.
It's fascinating that the same word describes both an emotional state (wearing yourself down with worry) and a guitar part—both involve the idea of something being divided or worn! The musical meaning is relatively modern compared to the worry meaning.
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