“So in pacifist route you want to fuck the fish while in the Genocide route you fuck the fish up.”
— Joel, talking about Undyne

Undyne is Joel's wish girlfriend.

Undyne is a fish monster from the a game called Undertale. When seeing sexy fan art of Undyne Joel would say something on the lines of "Fuck the fish", "A good boy sleeps with his hands above the sheets" or just doing a Pop noise.