An idiomatic expression describing a sour, sullen, or displeased expression — literally 'as if having chewed and crushed a bitter insect.'
昨日の会議のあと、彼は苦虫を噛み潰したような顔でデスクに戻った。
After yesterday's meeting, he returned to his desk with a sour look on his face.
先生に叱られて、生徒が苦虫を噛み潰したように黙り込んだ。
Scolded by the teacher, the student fell silent with a sullen expression.
ニュースを聞いた彼女は苦虫を噛み潰したような表情を浮かべた。
Upon hearing the news, she wore a discontented, sour expression.