noun
Vulgar slang for a woman who lies still and unresponsive during sex, likened to a dead tuna. The English glosses 'dead fish' and 'starfish' are approximate equivalents.
See also: マグロ
あの子、マグロ女らしいよ。
I heard that girl is a dead fish in bed.
彼女がマグロ女だなんて、ちょっとショックだった。
I was a bit shocked to find out she's a starfish.
マグロ alone can also be used as slang for an unresponsive sexual partner, but マグロ女 specifically refers to a woman.
Compound of マグロ (tuna) and 女 (woman), from the image of a dead tuna lying motionless.