When a guy is titty-fucking a girl, and when he cums, he cums on her chin and gives her a beard (like Gandalf).
A: "Hey man, guess what? I'm about to go Gandalfing"
B: "You're about to do what?!"
A: "Gandalfing. I'm about to go titty-fuck a girl, cum on her chin, and give her a beard... like Gandalf!"
B: "Dude, nice!"