gnaw v.

Old English gnæt "gnat, midge, small flying insect," earlier gneat, from Proto-Germanic *gnattaz (source also of Low German gnatte, German Gnitze); perhaps literally "biting insect" and related to gnaw.

The gnatte is a litil fflye, and hatte culex he soukeþ blood and haþ in his mouþ a pipe, as hit were a pricke. And is a-countid a-mong volatiles and greueþ slepinge men wiþ noyse & wiþ bytinge and wakeþ hem of here reste. [Bartholomew Glanville, "De proprietatibus rerum," c. 1240, translated by John of Trevisa c. 1398 ]

Gnat-catcher, insectivorous bird of the U.S. woodlands, is from 1823.

1828, intransitive, "find fault constantly;" by 1840, intransitive, "annoy by continued scolding, pester with petty complaints," originally a dialectal word meaning "to gnaw" (1825, Halliwell), probably ultimately from a Scandinavian source (compare Old Norse gnaga "to complain," literally "to bite, gnaw," dialectal Swedish and Norwegian nagga "to gnaw"), from Proto-Germanic *gnagan, related to Old English gnagan "to gnaw" (see gnaw). As a noun, 1894, "act of nagging;" by 1925, "person who nags." Related: Nagged; nagger; nagging.