When someone says "Shit's beat," it is usually referring to a girl being easy or her vagina being loose.
1. When talking about a slutty girl: "Dude, shit's beat!"
2. After having sex with a girl and your friends ask for details: "Shit's beat!"
3. After having sex with a SKANKY girl and your friends ask for details: "Shit's BEEN beat, man."
4. A reply from someone towards a girl (not necessarily a slutty one) asking a question: "So, how do I look?" "Shit's beat!"