Meaning of “Maguro(マグロ)”

“Maguro(マグロ)” means tuna, but as a slang, it has two different meanings.
One is dead body hit by a train. The other is a person who is motionless during sex.
Both slang meanings derive from the image of dead motionless tuna.

Example Conversation

A: How’s it going with your girlfriend?
B: Well, she’s cute, but she’s “Maguro(マグロ)”

A: 彼女と付き合ってみてどう？
B: いやー、かわいいんだけどマグロなんだよね。