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"

The Free On-line Dictionary of Computing (19 January 2023):

NAG

   1. Numerical Algorithms Group.

   2. The Linux Network Administrators' Guide.