The phrase "be delusional" refers to a state of having false beliefs or perceptions that are not based in reality. When someone is delusional, they may hold onto ideas or convictions that are strongly opposed to factual evidence or common understanding, often due to a psychological condition or intense emotion.
The word "delusional" comes from the Latin root "deludere," which means "to deceive or mock." It combines the prefix "de-" meaning 'down from' or 'away,' and "ludere," which means 'to play.' The notion is thus associated with being led away from reality, often through self-deception or false beliefs.