expression, noun
stern expression; severe expression; grim expression
Describes a facial expression that looks harsh, strict, or forbidding. Often used when someone is angry, serious, or intimidating.
彼は険しい表情で会議に臨んだ。
He attended the meeting with a stern expression.
先生の険しい表情を見て、生徒たちは静かになった。
Seeing the teacher's severe expression, the students fell silent.
怖い顔 means 'scary face' and emphasizes fearfulness, whereas 険しい表情 focuses on severity or grimness without necessarily being frightening.
Compound of 険しい (steep, severe, grim) and 表情 (facial expression). The exact historical derivation is uncertain, but the phrase is a straightforward combination of its components.