もっともらしい顔かお

expression

dignified expression; solemn look; serious face

Describes a facial expression that appears dignified, solemn, or serious, often with a nuance of putting on an air of importance or gravity. The phrase もっともらしい itself means 'plausible' or 'specious', so the expression can imply a feigned seriousness.

• 彼かれはもっともらしい顔かおでうそをついた。

  He told a lie with a straight face.

• 会かい議ぎ中ちゅう、部ぶ長ちょうはずっともっともらしい顔かおをしていた。

  During the meeting, the department head kept a solemn expression the whole time.