1. lacking physical substance or solidity; flimsy or weak
2. lacking importance, significance, or worth; trivial
3. lacking adequate evidence or basis; groundless

Examples

• The old shed was too insubstantial to withstand the storm.
• Her argument was insubstantial and failed to convince anyone.
• The witness provided only insubstantial evidence for his claims.
• The fabric was so insubstantial that it tore easily.
• Critics dismissed the proposal as insubstantial and poorly researched.
• The ghost appeared as an insubstantial figure in the moonlight.
• His concerns about the project seemed insubstantial to the committee.