Old English gnagan "to gnaw, bite off little by little" (past tense *gnog, past participle gnagan), from Proto-Germanic *gh(e)n- "to gnaw" (source also of Old Saxon gnagan, Old Norse, Swedish gnaga, Middle Dutch, Dutch knagen, Old High German gnagan, German nagen "to gnaw"), probably imitative of gnawing. Figurative sense "wear away as if by continued biting" is from early 13c. Related: Gnawed; gnawing.
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