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.
彼はもっともらしい顔でうそをついた。
He told a lie with a straight face.
会議中、部長はずっともっともらしい顔をしていた。
During the meeting, the department head kept a solemn expression the whole time.
An adjective meaning 'plausible' or 'specious'; もっともらしい顔 is a noun phrase built from it, focusing on the facial expression.
Means 'serious face' but without the nuance of pretense; 真面目 implies genuine earnestness, while もっともらしい顔 can suggest a put-on seriousness.
Derived from the adjective もっともらしい (plausible, specious) combined with 顔 (face). The adjective itself comes from もっとも (most, quite right) + らしい (seeming), literally 'seeming quite right'.