Adjective. Extremely old and antiquated, especially of behaviour or appearance. "??When Rip Van Winkle returned after waking from his long sleep, the villagers were struck dumb by his antediluvian appearance.?? Antediluvian implies old or antiquated in appearance or behavior. The sentence given above implies that Rip Van Winkle, who came after a long time, looked old. His attire must have looked antique too; the kind of clothes people wore too must have changed over the years."