From 澄(す)まし (sumashi, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 澄(す)ます (sumasu).”) + 顔(かお) (kao). The kao changes to gao as an instance of rendaku (連濁).
すましがお • (sumashigao) ←すましがほ (sumasigafo)?