The Collaborative International Dictionary of English v.0.48:

Nagging \Nag"ging\ (n[a^]g"g[i^]ng), a.
   Fault-finding; teasing; persistently annoying; as, a nagging
   toothache. [Colloq.]
   [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]

WordNet (r) 3.0 (2006):

nagging
    adj 1: continually complaining or faultfinding; "a shrewish
           wife"; "nagging parents" [syn: shrewish, nagging]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "nagging":
   adverse criticism, animadversion, aspersion, bad notices,
   bad press, blandishment, buttonholing, cajolement, cajolery,
   cajoling, captious, captiousness, carping, catty, cavil, caviling,
   censoriousness, chronic, coaxing, continual, continuous, critical,
   criticism, cynical, demanding, distressing, dunning, exception,
   faultfinding, flak, fretful, hairsplitting, haunting, hit,
   home thrust, hostile criticism, hypercritical, hypercriticalness,
   hypercriticism, importunate, importunateness, importunity,
   imputation, indelible, insistent, knock, naggy, niggle, niggling,
   nit, nit-picking, obloquy, obsessive, overcritical,
   overcriticalness, peevish, persistent, pestering, pettifogging,
   pettish, petulant, plaguing, plying, pressing, pressure,
   priggishness, querulous, quibble, quibbling, rap, recurrent,
   recurring, reflection, relentless, reproachfulness, resentful,
   shrewish, slam, stricture, swipe, taking exception, teasing,
   trichoschistic, trichoschistism, ultracritical, unforgettable,
   unrelenting, urgency, urgent, urging, vixenish, vixenly,
   wheedling