When a guy is really quick in bed
or
When you should only be getting to third base, but based on your skill, luck or some mistake, you get laid.
Girl: Wow, 5 seconds. Is that it?
Boy: Yeah, in the park home run. Sorry.
or
Wow, I totally wasn't expectingto get laid.
By Libbey
Inside The Park Home Run
Having sexual relations in the bathroom of a bar. Called the inside the park home run because you're hooking up right there and then in the bar, whilst saving yourself the trouble of traveling to the others domicile.
Bro #1: "Dude I totally ended last night with an Inside-the-park Home run."
Bro #2: "Isn't that illegal?"
Bro #1: "Not when the catcher's drunk!"
Bro #2: "Chyeah, brah!"
From American Dad, season 12, episode 20, "Gift Me Liberty":
Stan: "I know you're hiding sonething!"
Dick: "(Sobs.) OK! OK! I'm the one taking dumps in the urinals. I call them 'inside-the-park home run'."