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.

"old horse," c. 1400, nagge "small riding horse, pony," a word 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. The term of abuse "a worthless person," often of a woman, is a transferred sense, first recorded 1590s. For "one who annoys by scolding" (by 1925) see nag (v.).

in Hindu mythology, a race of serpent demons, offspring of Kaduru, guardians of the under-regions; 1785, from Sanskrit naga "serpent, snake," a word of unknown origin.

native people of southern Mexico and Central America, including the Aztecs; also their language, 1822, from Spanish, from Nahuatl (Aztecan) Nahuatl, the people's name, "something that makes an agreeable sound; someone who speaks well or speaks one's own language." As a language name, it was usually in the compound form nahuatlahotol-li.

in Greek and Roman mythology, "water nymph," one of the female deities presiding over springs and streams, c. 1600, from Latin Nais, Naias (genitive naiadis), from Greek Naias (plural Naiades) "river nymph," from naiein "to flow," from PIE *naw-yo-, suffixed form of *(s)nau- "to swim, flow, let flow" (from PIE root *sna- "to swim"). Dryden used the Latin singular form Nais, and the plural Naiades is attested in English from late 14c.

"ingenuous, artless, natural," 1590s, from French naïf, literally "naive" (see naive). The masculine form of the French word, but used in English without reference to gender. As a noun, "natural, artless, naive person," first attested 1893, from French, where Old French naif also meant "native inhabitant; simpleton, natural fool."

Old English næglian "to fix or fasten (something) onto (something else) with nails," from Proto-Germanic *ganaglijan (source also of Old Saxon neglian, Old Norse negla, Old High German negilen, German nageln, Gothic ganagljan "to nail"), from the root of nail (n.). Related: Nailed; nailing. The colloquial meaning "secure, succeed in catching or getting hold of (someone or something)" is by 1760; hence "to arrest" (by 1930). Meaning "to succeed in hitting" is from 1886. To nail down "to fix down with nails" is from 1660s.

Old English negel "tapering metal pin," nægl "fingernail (handnægl), toenail," from Proto-Germanic *naglaz (source also of Old Norse nagl "fingernail," nagli "metal nail;" Old Saxon and Old High German nagel, Old Frisian neil, Middle Dutch naghel, Dutch nagel, German Nagel "fingernail; small metal spike"), from PIE root *(o)nogh "nail of the finger or toe" (source also of Greek onyx "claw, fingernail;" Latin unguis "fingernail, claw;" Old Church Slavonic noga "foot," noguti "fingernail, claw;" Lithuanian naga "hoof," nagutis "fingernail;" Old Irish ingen, Old Welsh eguin "fingernail, claw").

The "fingernail" sense seems to be the original one, but many figurative uses are from the "small metal spike" sense: hard as nails is from 1828. To hit the nail on the head "say or do just the right thing" is by 1520s; in Middle English driven in the nail (c. 1400) was "to drive home one's point, clinch an argument," and smiten the nail on the hed was "tell the exact truth" (mid-15c.). Phrase on the nail "on the spot, exactly" is from 1590s, of obscure origin; OED says it is not certain it belongs to this sense of nail.

As a unit of English cloth measure (about 2 1/4 inches) from late 14c.; perhaps from a nail being used to mark that length on the end of a yardstick.