Added 9 October 2026 nightmarish adjective Categories: Describing WordsRare Words adjective ! Definition from WordNet 3.1, Princeton University. More about nightmarish at Wordnik → Where our words come from → Extremely alarming. Related Words: bloodcurdlinghair-raisingalarmingfearsometerrifyingfrighteningappallingdistressingdepressingnightmarelikenightmariousdreamish Back to today's word