insubstantial

Probably from Middle French insubstantiel, from Latin insubstantiālis, from in- + substantiālis. By surface analysis, in- +‎ substantial.

insubstantial (comparative more insubstantial, superlative most insubstantial)

1. Lacking substance; not real or strong.

   The bridge was insubstantial and would not safely carry a car.

   • 2008, Christopher Paolini, “Rider and Ra'zac”, in Brisingr, pages 67-8:

     By manipulating the flow of magic, Eragon quickly descended from the Ra'zac's lair—which the insubstantial wall of stone once again hid—to the ledge.

• unsubstantial (archaic); See also Thesaurus:insubstantial

• substantial; See also Thesaurus:substantial