noun
straight face; composed look; nonchalant expression
Describes a face that shows no emotion, often deliberately calm or pretending not to notice something. Compare with すまし顔, which can also mean 'prim face' or 'smug look'.
See also: すまし顔
彼は怒られているのに、すました顔をしている。
He's being scolded, but he keeps a straight face.
いたずらをした後、猫はすました顔で座っていた。
After the mischief, the cat sat with a nonchalant look.
Similar meaning, but すまし顔 can also imply a prim, smug, or self-satisfied expression, while すました顔 focuses more on a blank or composed look.
Compound of 澄ました (past form of 澄ます 'to clear, to look composed') + 顔 'face'. The exact historical derivation is straightforward.