"I got to second base with that girl yesterday." "I didn't think I had a chance, but I made it to third base with her last night." One of the other answers has provided a nice link to the Wikipedia entry for baseball metaphors for sex.The sexual contact associated with each base has evolved dramatically since I was a teenager, although "home base" or "home run" has always meant intercourse, and "first base" has always meant kissing or snogging.But this is a joke, and probably not intended to be analyzed too closely. Although this wasn't what "House" referred to, Urban Dictionary lists fifth base as anal sex.Talking about fifth base when there's really only four bases reminds me of the expression Up To Eleven.
Let’s leave blatant sex talk to the French, shall we? (Note: Probably the least celebrated and referred-to base ever.) Third Base: Fondling…with your mouth. Fifth Base: How to put this delicately…the, uh, final frontier.
Effectively, I'm asking for equivalent metaphors in other English dialects.
Note that, in the baseball metaphor, there is an implicit "escalation" idea baked into the metaphor.
Although it's a minority sport today, it's still widely played in British primary schools because everybody gets a turn.
There are, as the top comment on this question notes, four bases in baseball, and these have corresponding sex acts associated with them.