A fish loaf is a girl who just lays there during sex. No movement. no sound. Just a fish loaf. You might as well be having sex with a giant loaf of fish because that would be about as exciting as banging a girl who is a "fish loaf". If any girl openly admits to not liking sex very much. There is a good chance she is a fish loaf.
Tom:"I finally got Jenna into bed with me last night and she was so boring"
Ted:"What do you mean?"
Tom:"She just laid there and didn't make a sound!"
Ted:"Oh so she's a FISH LOAF. Haha. That sucks."