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
He's being scolded, but he keeps a straight face.
After the mischief, the cat sat with a nonchalant look.
Kana spelling is common in everyday writing, especially when the kanji 澄 is less familiar.
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.