When a guy has a dirty penis or one that is "misshapen" in any way. Also, guys who have smegma.
1) Sarah: "Oh my god! I was going to give Wes a blowjob and his dick was crusty!"
2) Vince: "Dude, look at your shit-dick, what the fuck is wrong with it?"
Charles: "I don't know do the head is weird looking. I'm going to go get it checked out"
Vince: "That's probably a good idea Mr. Shit-dick."