Annoyed, worried, or irritated; also used to describe something that has grooves or ridges cut into it like a guitar fretboard.
From Old English 'fretan' meaning to devour or gnaw. The sense of worry evolved metaphorically from the idea of something gnawing at the mind, while the musical/decorative sense comes from the pattern of frets on instruments.
It's wild that the same word describes both an emotional state and a physical pattern—English used the image of something 'gnawing' at you to capture anxiety, which is why we still say worry 'eats at us' today.
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