May 14, 2026 · The meaning of MYSTERIOUS is of, relating to, or constituting mystery. How to use mysterious in a sentence. The costume is mysterious in its poverty as well as in its flair for incoherent details. MYSTERIOUS definition: full of, characterized by, or involving mystery. See examples of mysterious used in a sentence. Someone or something that is mysterious is strange and is not known about or understood. He died in mysterious circumstances. A mysterious illness confined him to bed for over a month. involving or full of mystery: a mysterious phone call. suggesting or implying a mystery: a mysterious smile. puzzling; inexplicable: a mysterious inscription on an ancient tomb. On an island in the Bermuda triangle,a film producer cannot resist the siren call when he sees a dolphin push a beautiful, unconscious woman ashore. Definition of mysterious adjective in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more. The comprehensive definition of mysterious.