"annoy by scolding," 1828, originally a dialectal word meaning "to gnaw" (1825), probably ultimately from a Scandinavian source (cf. 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). Related: Nagged; nagger; nagging.
"old horse," c.1400, nagge "small riding horse," of unknown origin, perhaps related to Dutch negge, neg (but these are more recent than the English word), perhaps related in either case to imitative neigh. Term of abuse is a transferred sense, first recorded 1590s.
A horse, esp an old and worn-out racehorse: to make dough on the nags
[1400+; origin unknown]