The Collaborative International Dictionary of English v.0.48:

Tenuous \Ten"u*ous\, a. [L. tenuis thin. See Thin, and cf.
   Tenuis.]
   1. Thin; slender; small; minute.
      [1913 Webster]

   2. Rare; subtile; not dense; -- said of fluids.
      [1913 Webster]
      [1913 Webster]

WordNet (r) 3.0 (2006):

tenuous
    adj 1: having thin consistency; "a tenuous fluid"
    2: very thin in gauge or diameter; "a tenuous thread"
    3: lacking substance or significance; "slight evidence"; "a
       tenuous argument"; "a thin plot"; a fragile claim to fame"
       [syn: flimsy, fragile, slight, tenuous, thin]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "tenuous":
   adulterated, aerial, air-built, airy, aloof, atomic, attenuate,
   attenuated, boyish, broken, chimerical, cloud-built, corpuscular,
   cut, dainty, delicate, detached, diaphanous, dilute, diluted,
   disconnected, discontinuous, discrete, doubtful, dreamlike,
   dubious, embryonic, ethereal, evanescent, fanciful, fatuitous,
   fatuous, feeble, fine, fine-drawn, finespun, flimsy, fragile,
   frail, gapped, gaseous, gauzy, germinal, girlish, gossamer,
   gossamery, gracile, granular, hazy, illusory, imaginary,
   impalpable, imperceptible, imponderable, inappreciable, incoherent,
   inconsistent, indiscernible, infinitesimal, insignificant,
   insubstantial, intangible, invisible, lacy, light, microcosmic,
   microscopic, misty, molecular, nebulous, negligible, nonadherent,
   nonadhesive, noncoherent, noncohesive, open, paltry, papery,
   phantomlike, rare, rarefied, reedy, shadowy, shaky, sketchy,
   slender, slenderish, slight, slight-made, slim, slimmish, slinky,
   small, spirituous, subatomic, subtile, subtle, svelte, sylphlike,
   thin, thin-bodied, thin-set, thin-spun, thinned, thinned-out,
   thinnish, threadlike, trifling, twiggy, ultramicroscopic,
   unadhesive, uncoherent, uncohesive, uncompact, uncompressed,
   unconnected, unjoined, unreal, unseeable, unsubstantial,
   untenacious, vague, vaporous, wasp-waisted, watered, watered-down,
   watery, weak, willowy, windy, wiredrawn, wispy