1.
to frown, to knit one's brows
To draw the eyebrows together in an expression of displeasure, concern, or disapproval.
Structure: 眉 (eyebrow) + を + ひそめる (to draw together, to contract). ひそめる occurs almost only in this phrase.
Nuance: Silent disapproval or concern — the reaction to something improper, worrying, or distasteful. It is a description of how others react, not an accusation, so it is a common way to report disapproval without criticizing directly. The person frowning is usually a bystander.
Register: Slightly literary. It appears far more often in writing and in narration than in everyday speech, where 顔をしかめる is the ordinary way to say the same thing.