Delusional (adjective)

1. Having false or unrealistic beliefs or opinions.
2. Suffering from a condition in which a person holds a belief that is clearly false and that indicates an abnormal state of mind.

Origin:

Delusional comes from the latin word "deludere" which means "to mock" or "to deceive" and it was adopted in english in the late 19th century.

Examples:

1. He is delusional if he thinks he can win the election.
2. She was delusional about her abilities as a singer.
3. He was delusional when he thought he could fly.
4. The patient was delusional and suffering from hallucinations.
5. He was delusional in his belief that he was a millionaire.