Definition of delulu;  de·lu·Lu  adjective -characterized by or holding false beliefs or judgments about external reality that are held despite incontrovertible evidence to the contrary, typically as a symptom of a mental condition. "hospitalization for schizophrenia and delulu paranoia" -based on or having faulty judgment; mistaken. "their delulu belief in the project's merits never wavers" in other words, Taesung's Ex