biting or chewing repeatedly on something, or a persistent worry that bothers you continuously.
From Old English 'gnagan,' possibly related to Old Norse 'gnagg,' meaning 'to gnaw.' The word is onomatopoetic, imitating the sound of chewing.
The word 'gnaw' is almost certainly onomatopoetic—it imitates the sound a rat makes chewing! Humans use it metaphorically too; we talk about a 'gnawing doubt' because both physical gnawing and emotional worry are persistent and wearing.
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