Is insane an adjective or noun?

"Insane" can be both an adjective and a noun, depending on how it is used:

Adjective:

* Describes a state of mind: "He was declared legally insane."

* Describes something as extremely foolish or irrational: "That's an insane idea!"

Noun:

* Refers to a person with a mental disorder: "The insane were once treated poorly."

* Refers to a state of madness or extreme unreasonableness: "His actions were driven by sheer insane."

So, the answer is both. The meaning will be determined by the context of the sentence.