third base

The second to last base in baseball this is to the left of the batter.
Also called the hot corner. Take it as you will.

"All you need to play third base is a strong arm and a strong chest."

- Frankie Frisch, Cardinal infielder of the 30s (note that he was with the Cardinals, not a Cardinal. I'd pay good money to see a Cardinal play baseball).

Okay, if the person you are taking to is taking about sex and is in high school, chances are that he's making a reference to the petting/fingering of one's partner's pubic region, or, rarely, even in these more hedonistic days, oral sex.

Now, if he is actually talking about baseball, don't you feel silly?