1. lacking physical substance or solidity; not real or tangible
2. lacking strength, importance, or validity; weak or flimsy

Examples

• The ghost appeared as an unsubstantial figure in the moonlight.
• Her argument was unsubstantial and easily refuted.
• The morning mist created unsubstantial shapes in the garden.
• His claims were based on unsubstantial evidence.
• The fabric was so thin it felt almost unsubstantial.
• The committee found the proposal unsubstantial and rejected it.
• Smoke formed unsubstantial patterns in the air.