serious look; straight face
A facial expression that is earnest, unsmiling, or without joking; often used when someone suddenly becomes serious or when a joke is delivered with a deadpan expression.
He told a joke with a straight face.
Suddenly he became serious and started talking about something important.
He said that joke with a completely straight face.
She suddenly turned serious and looked at me.
The teacher asked that question with a serious expression.
無表情 means blank or expressionless, showing no emotion at all, whereas 真顔 implies a deliberately earnest or unsmiling look that can still convey seriousness.
笑顔 is a smiling face, the direct opposite of 真顔's unsmiling, serious expression.
素顔 means one's bare face without makeup or a mask, while 真顔 is about a serious facial expression, not the state of the skin.
Compound of 真 (ma, 'true') and 顔 (kao, 'face'), with rendaku voicing of かお to がお. The word straightforwardly means 'true face' in the sense of a serious, unfeigned expression.