Meaning

noun someone (especially a woman) who annoys people by constantly finding fault

noun an old or over-worked horse

verb bother persistently with trivial complaints

originProbably from a North Germanic source; compare Swedish nagga (“to gnaw, grumble”), Danish nage (“to nag, bother”), Icelandic nagga (“to complain”). Compare typologically fret, Bulgarian глождя (gloždja), Russian глода́ть (glodátʹ), грызть (gryztʹ), по́едом есть (pójedom jestʹ), е́дкий (jédkij).

Etymology from Wiktionary, CC BY-SA 3.0