Gnaw is Old English gnagan, "to bite off little by little," with relatives across Germanic — German nagen, Dutch knagen, Swedish gnaga. The word is probably imitative, an attempt to write the sound of the action, which is a common origin for words of this kind and helps explain why the awkward gn- cluster survived at all.
Like gnat and knee, gnaw preserves in spelling a consonant English stopped pronouncing around the 1600s. Before that, the g was really there. The cluster survives in the spelling because print had fixed English orthography by the time the sound went, which is the single most useful fact about English spelling: the system was frozen at the end of the 1400s, and the language kept moving underneath it. The figurative sense of wearing something away as if by biting is already there in the early 1200s.
gnaw (v.) 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 .
gnaw came in through Old English, and its spelling follows that family's conventions.
How that family writes these sounds
| /n/ at the start | kn- or gn-. The first letter is silent now and was not in 1400knee, knight, gnaw, gnat |
Old English in the spelling guide · guess another word's family