bitter-tasting bug; imaginary bug that tastes bitter
Refers to an imaginary insect with a bitter taste. Almost exclusively used in the set phrase 苦虫を噛み潰したよう (like biting a bitter bug), describing a sour, disgusted, or grimacing facial expression. The word itself is rarely used outside this idiom.
See also: 苦虫を噛み潰したよう (にがむしをかみつぶしたよう)
He heard the news with a look on his face as if he had bitten into a bitter bug.
The word 苦虫 refers not to a real insect but to an imaginary bug that tastes bitter.