The Collaborative International Dictionary of English v.0.48:

Nugatory \Nu"ga*to*ry\, a. [L. nugatorius, fr. nugari to trifle,
   nugae jests, trifles.]
   1. Trifling; vain; futile; insignificant.
      [1913 Webster]

   2. Of no force; inoperative; ineffectual.
      [1913 Webster]

            If all are pardoned, and pardoned as a mere act of
            clemency, the very substance of government is made
            nugatory.                             --I. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nugatory
    adj 1: of no real value; "a nugatory law"

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "nugatory":
   NG, airy, asinine, barren, bootless, catchpenny, cheap,
   counterproductive, effete, empty, etiolated, fatuitous, fatuous,
   feckless, flimsy, foolish, fribble, fribbling, frivolous, frothy,
   fruitless, futile, good-for-naught, good-for-nothing, hollow, idle,
   inadequate, inane, ineffective, ineffectual, inefficacious,
   inoperative, invalid, junk, junky, light, no-account, no-good,
   not worth having, not worth mentioning, not worthwhile, nugacious,
   of no force, otiose, shabby, shallow, shoddy, silly, slender,
   slight, sterile, superficial, trashy, trifling, trite, trivial,
   unavailing, useless, vacuous, vain, valueless, vapid, windy,
   worthless