one eyebrow
Refers to a single eyebrow, often used in contexts describing facial expressions or asymmetry.
彼は片眉を上げて驚いた表情をした。
He raised one eyebrow in surprise.
片眉だけを動かすのは難しい。
It's difficult to move just one eyebrow.
Compound of 片 (kata, 'one side') and 眉 (mayu, 'eyebrow'). The reading かたまゆ is a straightforward combination.