WordWeb Online

1. Lacking in nutritive value
   "an insubstantial and unsatisfying meal"
    
2. Lacking solidity or strength
   "vinyl siding has become the standard-bearer for cheap, insubstantial construction";
   - flimsy
    
3. Lacking material form or substance; unreal
   "an insubstantial mirage on the horizon"; "as insubstantial as a dream";
   - unsubstantial, unreal

See also: aerial, aeriform, aery, airy, ethereal, etherial, hollow, immaterial, nonmaterial, shadowy, solidness, stringy, substantiality, substantialness, unwholesome, weak, wraithlike