1.
looking sour; bitter-faced; looking extremely displeased
An idiomatic expression describing a facial expression of extreme displeasure, irritation, or bitterness, as if one had bitten down on something intolerably bitter.
Literally "as if one had crushed a bitter bug in one's teeth." 苦虫 is not a real insect — it is an imaginary creature used only in this idiom to evoke an intensely bitter taste. The expression vividly captures the tightened lips and furrowed brow of someone deeply displeased.
Usage
Almost always used in the pattern 苦虫を噛み潰したような + noun (especially 顔 or 表情). Describes suppressed displeasure — the person is unhappy but not shouting or crying; instead, the emotion shows silently in their face.