Insubstantial means lacking solidity, strength, or real importance. When something is insubstantial, it's flimsy, weak, or doesn't amount to much.

A spider's web is insubstantial: beautiful and intricate, but so delicate that a strong breeze can destroy it. A poorly built tree house made from thin branches might be too insubstantial to hold anyone's weight safely. When morning mist drifts across a field, it seems insubstantial, almost like you could walk right through it without feeling anything.

The word also describes ideas or arguments that don't hold up under examination. If you claim your dog ate your homework but can't explain how the dog got into your closed backpack, your teacher might say your excuse is insubstantial. When a lawyer presents insubstantial evidence in court, the case falls apart because there's nothing solid backing it up.

Think of insubstantial as the opposite of substantial, which means solid and significant. A substantial bridge can support heavy trucks; an insubstantial one might collapse under a bicycle. A substantial meal fills you up; an insubstantial snack leaves you hungry an hour later. When something matters and has real weight, whether physical or otherwise, it's substantial. When it's weak, thin, or unconvincing, it's insubstantial.