i-adjective
plausible; believable; specious
Describes something that sounds or looks reasonable on the surface, often with an implication that it may not be entirely true or sincere.
彼の言い訳はもっともらしかったが、誰も信じなかった。
His excuse sounded plausible, but nobody believed it.
もっともらしい理屈を並べて、彼は自分の失敗を正当化した。
He justified his failure by lining up specious arguments.
i-adjective
Used for a person's demeanor, expression, or tone that is grave, dignified, or pretending to be serious. Often carries a nuance of affected gravity.
彼はもっともらしい顔でうなずいた。
He nodded with a solemn look on his face.
もっともらしい口調で話し始めた。
He began to speak in a dignified tone.