Source : Webster's Revised Unabridged Dictionary (1913)

Nag \Nag\, n. [OE. nagge, D. negge; akin to E. neigh.]
   1. A small horse; a pony; hence, any horse.

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

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.

Source : WordNet®

nag
     n 1: someone (especially a woman) who annoys people by constantly
          finding fault [syn: {scold}, {scolder}, {nagger}, {common
          scold}]
     2: an old or over-worked horse [syn: {hack}, {jade}, {plug}]
     [also: {nagging}, {nagged}]

nag
     v 1: bother persistently with trivial complaints; "She nags her
          husband all day long" [syn: {peck}, {hen-peck}]
     2: worry persistently; "nagging concerns and doubts"
     3: remind or urge constantly; "she nagged to take a vacation"
     [also: {nagging}, {nagged}]

Source : Free On-Line Dictionary of Computing

NAG
     
        1. {Numerical Algorithms Group}.
     
        2. The {Linux Network Administrators' Guide}.