Constantly worried, irritable, or restless; characterized by nervous anxiety or impatience.
From 'fret' (Old English 'fretan,' meaning to eat away or gnaw; later to worry) plus '-ful' (suffix meaning full of or characterized by). The semantic shift from 'eating away' to 'worrying' reflects how anxiety eats at the mind.
The original meaning of 'fret' was literally 'to eat away,' and fretful preserves that metaphor—worry gnaws at you like a mouse gnawing food. That's why someone fretful seems worn down and anxious!
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