‘Insane' is adjective.
Opposite word of ‘insane' is ‘sane.'
Meaning: seriously mentally ill and unable to live in normal society.
1. The patient was certified as insane by the doctors.
2. The prison has a very bad environment. The prisoners were slowly going insane in this prison.
‘Insane' has an informal meaning: very stupid, crazy or dangerous.
1. I must have been insane to agree to the idea.
2. He is insanely jealous.