1.Definition: lacking in nutritive value

"An insubstantial and unsatisfying meal"

2.Definition: lacking material form or substance; unreal

"As insubstantial as a dream", "An insubstantial mirage on the horizon"

Related Adjective(s):unreal, unsubstantial

3.Definition: lacking solidity or strength

"Vinyl siding has become the standard-bearer for cheap, insubstantial construction"

Related Adjective(s):flimsy