"insubstantial" in the adjective sense
1. insubstantial, unsubstantial, unreal
lacking material form or substance unreal
"as insubstantial as a dream"
"an insubstantial mirage on the horizon"
2. insubstantial
lacking in nutritive value
"an insubstantial and unsatisfying meal"
3. flimsy, insubstantial
lacking solidity or strength
"a flimsy table"
"flimsy construction"
"vinyl siding has become the standard-bearer for cheap, insubstantial construction"
Source: WordNet® (An amazing lexical database of English)
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