The Collaborative International Dictionary of English v.0.48:

Nag \Nag\ (n[a^]g), n. [OE. nagge, D. negge; akin to E. neigh.]
   1. A small horse; a pony; hence, any horse, especially one
      that is of inferior breeding or useless.
      [1913 Webster]

   2. A paramour; -- in contempt. [Obs.] --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nag \Nag\, v. t. & i. [imp. & p. p. Nagged; p. pr. & vb. n.
   Nagging.] [Cf. Sw. nagga to nibble, peck, Dan. nage to
   gnaw, Icel. naga, gnaga, G. nagen, & E. gnaw.]
   To tease in a petty way; to scold habitually; to annoy; to
   fret pertinaciously. [Colloq.] "She never nagged." --J.
   Ingelow.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Nag \Nag\ (n[a^]g), n.
   A person who nags, especially habitually; called also
   nagger.
   [PJC]

WordNet (r) 3.0 (2006):

nag
    n 1: someone (especially a woman) who annoys people by
         constantly finding fault [syn: scold, scolder, nag,
         nagger, common scold]
    2: an old or over-worked horse [syn: hack, jade, nag,
       plug]
    v 1: bother persistently with trivial complaints; "She nags her
         husband all day long" [syn: nag, peck, hen-peck]
    2: worry persistently; "nagging concerns and doubts"
    3: remind or urge constantly; "she nagged to take a vacation"

Moby Thesaurus II by Grady Ward, 1.0:

169 Moby Thesaurus words for "nag":
   Houyhnhnm, advocate, aggravate, annoy, annoyer, apply pressure,
   badger, badgerer, bait, balker, balky horse, be at, bedevil, beset,
   besiege, blandish, bother, bring to recollection, bristle,
   brood mare, brown off, bug, bully, bullyrag, burn up, buttonhole,
   cajole, call on, call upon, carp at, carry back, charger, chivy,
   coax, colt, courser, critter, crock, crowbait, devil, discompose,
   distemper, disturb, dobbin, dog, dun, egg, entire, entire horse,
   equine, exasperate, exercise, exert pressure, exhort, fash, filly,
   foal, fret at, fuss, fuss at, garron, gelding, get, give a hint,
   give the cue, goad, goat, gripe, hack, harass, harasser, harrier,
   harry, hassle, heckle, heckler, hector, henpeck, high-pressure,
   hold the promptbook, horse, hound, importune, insist, insist upon,
   irk, jade, jawbone, jog the memory, jughead, lobby, mare, miff,
   molest, mount, nag at, needle, nettle, nibble at, niggle, nudge,
   nudnik, nudzh, peck at, peeve, persecute, persecutor, pest, pester,
   pesterer, pick at, pick on, pique, plague, plaguer, plead with,
   pluck the beard, plug, ply, pother, prancer, press, pressure, prod,
   prompt, prompt the mind, provoke, push, put in mind,
   put in remembrance, recall, recommend, remember, remind,
   remind one of, ride, rile, roarer, rogue, roil, rosinante, ruffle,
   sadist, scalawag, soft-soap, stallion, steed, stiff, stud,
   studhorse, suggest, sweet-talk, tarpan, tease, teaser, top horse,
   torment, tormentor, try the patience, tweak the nose, urge, vex,
   war-horse, wheedle, whistler, wild horse, work on, worry, yap at



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NAG
       Numerical Algorithms Group (UK, org.), "NAg"


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NAG
       National Algorithms Group [ltd] (UK, org., predecessor), "NAg"


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NAG
       Network Architecture Group (org.)


The Free On-line Dictionary of Computing (30 December 2018):

NAG

   1. Numerical Algorithms Group.

   2. The Linux Network Administrators' Guide.