Open English Wordnet
Adjectives
(a)insubstantial, unsubstantial, unreallacking material form or substance; unreal“as insubstantial as a dream”, “an insubstantial mirage on the horizon”
(s)flimsy, insubstantiallacking solidity or strength“a flimsy table”, “flimsy construction”, “vinyl siding has become the standard-bearer for cheap, insubstantial construction”