I always hear people say “I hit the third base” or “I hit the second base” (sex related). I am not 100% sure what they all mean.
Additionally, in one of the House MD episodes, there was a dialogue:
Dr. Wilson: How are things going with Cuddy?
House: Great. We’ve gotten to fifth base. That’s two home runs, and then she gives me back a triple.
In one of the American Dad episodes, I remember Steve saying something along the lines, “did I just hit the second base?” after holding hands with a girl.