1.adjectival nouns or quasi-adjectives (keiyodoshi); nouns which may take the genitive case particle 'no'
smooth; flat; lacking bumps and dents
2.adjectival nouns or quasi-adjectives (keiyodoshi); nouns which may take the genitive case particle 'no'
featureless; uneventful
3.noun (common) (futsuumeishi)
noppera-bō; mythical being with flat featureless face