The Collaborative International Dictionary of English v.0.48:

Negligible \Neg"li*gi*ble\, a. [Cf. F. n['e]gligible,
   n['e]gligeable.]
   That may be neglected, disregarded, or left out of
   consideration; too small or unimportant to be worthy of
   notice.
   [1913 Webster]

         Within very negligible limits of error.  --Sir J.
                                                  Herschel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

negligible
    adj 1: so small as to be meaningless; insignificant; "the effect
           was negligible"
    2: not worth considering; "he considered the prize too paltry
       for the lives it must cost"; "piffling efforts"; "a trifling
       matter" [syn: negligible, paltry, trifling]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "negligible":
   back-burner, cursory, depthless, dinky, dispensable, fat, few,
   footling, immaterial, inappreciable, inconsequential,
   inconsiderable, inessential, inferior, insignificant, irrelevant,
   little, low, meager, miniature, minor, minute, niggling,
   no great shakes, nonessential, not vital, nugatory, off, outside,
   paltry, petit, petty, picayune, picayunish, piddling, shallow,
   short, skin-deep, slender, slight, slim, small, superficial,
   technical, tiny, trifling, trivial, unessential, unimportant,
   unimpressive, unnoteworthy, worthless