a warning from guys to their guy friends when they see them talking to a girl who seems promiscuous or is a well-known slut, to keep them away from said girl, in reference to the girls vagina resembling an Arby's roast beef sandwich.
Sonny: "Hey, dude, I'm talking to that one Megan chick. She seems nice."
Hayden: "Dude, she's slept with so many guys. I'm thinkin' arby's."