noun
straight face; composed look; nonchalant expression
Describes a facial expression that deliberately hides one's true feelings, often appearing calm, indifferent, or slightly smug. Can be used positively for composure or negatively for feigned innocence.
彼は叱られてもすまし顔で立っていた。
Even when scolded, he stood there with a straight face.
いたずらをした猫がすまし顔で
The cat that caused mischief is sitting with a nonchalant look.
Compound of すまし (continuative form of 澄ます, 'to put on airs' or 'to look composed') and 顔 (かお, 'face'). The spelling 澄まし顔 reflects the verb 澄ます, while すまし顔 is the more common mixed kana-kanji form.