Name that fish

Hello,
Do you know the name of the fish where, if you eat it, the oil of the fish does not get digested in your system?

My friend and I sometimes talk about gross things, and I was telling her about this, but it's not as effective when I can't think of the name of the fish and then look it up on the Internet for more info.

If you know please post in the comments!

Thanks!
Bladio