From The Collaborative International Dictionary of English v.0.44 [gcide]:
Unsubstantial \Un'sub*stan"tial\, adjective Lacking in matter or substance; visionary; chimerical.
From WordNet (r) 2.0 [wn]:
adjective
1: lacking material form or substance; unreal; "as insubstantial as a dream"; "an insubstantial mirage on the horizon" [syn: {insubstantial}, {unreal}] [ant: {substantial}]
From Moby Thesaurus II by Grady Ward, 1.0 [moby-thes]:
182 Moby Thesaurus words for "unsubstantial": Barmecidal, Barmecide, adulterated, airy, apparent, apparitional, asomatous, astral, attenuate, attenuated, autistic, bodiless, breakable, brittle, cheap-jack, chimeric, cobwebby, crumbling, crumbly, cut, dainty, dangerous, decarnate, decarnated, deceptive, decrepit, delicate, delicately weak, delusional, delusionary, delusive, delusory, dereistic, desultory, dilute, diluted, discarnate, disembodied, disintegrating, dreamlike, dreamy, effeminate, erroneous, ethereal, extramundane, fallacious, false, fanciful, fantastic, feeble, fine, flimsy, fragile, frail, frangible, gaseous, ghostly, gimcrack, gimcracky, gossamery, hazardous, illusional, illusionary, illusive, illusory, imaginary, immaterial, impalpable, imponderable, improbable, inconceivable, inconclusive, incorporate, incorporeal, incredible, infirm, insecure, insubstantial, intangible, jerry, jerry-built, light, lightweight, merely nominal, metaphysical, misleading, namby-pamby, nonmaterial, nonphysical, nonsubstantial, not real, occult, ostensible, otherworldly, papery, pasteboardy, perilous, phantasmagoric, phantasmal, phantom, poor, poorish, precarious, provisional, psychic, puny, rare, rarefied, risky, rotten, rotten at, seeming, self-deceptive, self-deluding, shadowy, shaky, shattery, shifting, shifty, sissified, sleazy, slight, slippery, specious, spectral, spiritual, subtile, subtle, supernatural, supposititious, tacky, temporary, tentative, tenuous, thick, thin, thinned, thinned-out, ticklish, transmundane, treacherous, unactual, unbelievable, uncompact, uncompressed, unconcrete, unconvincing, undependable, unearthly, unembodied, unextended, unfaithworthy, unfirm, unfleshly, unfounded, unphysical, unproved, unreal, unrealistic, unreliable, unrigorous, unsolid, unsound, unstable, unsteadfast, unsteady, unsturdy, unsubstanced, unsure, unsustained, untrustworthy, unworldly, vaporous, visionary, watered, watered-down, weak, weakly, weightless, windy, wispy, womanish
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