The Collaborative International Dictionary of English v.0.48:

Insubstantial \In`sub*stan"tial\, a.
   Unsubstantial; not real or strong. "Insubstantial pageant."
   [R.] --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

insubstantial
    adj 1: lacking material form or substance; unreal; "as
           insubstantial as a dream"; "an insubstantial mirage on
           the horizon" [syn: insubstantial, unsubstantial,
           unreal] [ant: material, real, substantial]
    2: lacking in nutritive value; "the jejune diets of the very
       poor" [syn: insubstantial, jejune]

Moby Thesaurus II by Grady Ward, 1.0:

196 Moby Thesaurus words for "insubstantial":
   adulterated, airy, asomatous, astral, attenuate, attenuated,
   bodiless, boyish, brittle, capricious, changeable, chimerical,
   corruptible, cut, dangerous, decarnate, decarnated, deciduous,
   decrepit, defalcation, deficiency, deficit, delicate, desultory,
   diaphanous, dilute, diluted, discarnate, disembodied, dying,
   ephemeral, ethereal, evanescent, extramundane, fading, false,
   fancied, fanciful, fantastic, feeble, fickle, fine, fine-drawn,
   finespun, fleeting, flimsy, flitting, fly-by-night, flying,
   fragile, frail, fugacious, fugitive, gaseous, gauzy, ghostly,
   girlish, gossamer, gracile, hallucinatory, illusive, illusory,
   imaginary, imagined, immaterial, impalpable, impermanent,
   impetuous, imponderable, impulsive, inadequacy, inconclusive,
   inconstant, incorporate, incorporeal, infirm, insecure,
   insignificant, insufficiency, intangible, lack, lacy, light,
   metaphysical, misty, momentary, mortal, mutable, nondurable,
   nonmaterial, nonpermanent, nonphysical, nonsubstantial, occult,
   otherworldly, paltry, papery, passing, paucity, perilous,
   perishable, phantasmal, phantom, poor, poverty, precarious,
   provisional, psychic, puny, rare, rarefied, risky, scant,
   scantiness, scarceness, shadowy, shaky, shifting, shifty,
   short-lived, shortage, slender, slenderish, slight, slight-made,
   slim, slimmish, slinky, slippery, small, spiritual, subtile,
   subtle, supernatural, svelte, sylphlike, temporal, temporary,
   tentative, tenuous, thin, thin-bodied, thin-set, thin-spun,
   thinned, thinned-out, thinnish, threadlike, ticklish, transient,
   transitive, transitory, transmundane, treacherous, uncompact,
   uncompressed, unconcrete, undependable, underage, undurable,
   unearthly, unembodied, unenduring, unextended, unfaithworthy,
   unfleshly, unphysical, unproved, unreal, unreliable, unrigorous,
   unsolid, unsound, unstable, unsteadfast, unsteady, unsubstanced,
   unsubstantial, unsure, unsustained, untrustworthy, unworldly,
   vague, vaporous, visionary, volatile, wasp-waisted, watered,
   watered-down, watery, weak, weightless, willowy, windy, wiredrawn,
   wispy