Rock tit is what happens to your nipples when they get cold, or in some cases, when one gets horny. Rock tit is characterized by a sudden hardening of the nipples, hence the name.
More "severe" cases of rock tit, where ones nipples are so hard they could shatter bones, is named "Diamond tit".
Joe: Dude, you should have put on a better shirt for today.
Hal: Why's that?
Joe: It's cold out and I can see your rock tit.
Hal: Aww shit!