Entry 1 sense Webster, 1913 Somniferous · Som·nif·er·ous · IPA /sɑmˈnɪfəɹəs/ 01 a. Causing or inducing sleep; soporific; dormitive; as, a somniferous potion. 1. Causing or inducing sleep; soporific; dormitive; as, a somniferous potion.