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3.
The last of all bases. To turn yourself into a transexual and then rape your best friend and mother.
A- Sorry about your asshole I just wanted to get to 40th base.

B- I understand.
by ALo╛Prezidente July 17, 2008
 
1.
Two trains on the same track are moving towards each other, each at a speed of 40 miles per hour. On the top of one train stand 50 naked guys with boners. On the other train stand 50 naked girls who are horny. What happens to the ones that don't fall off when the two trains run into each other is called 40th base.
Damn, i just got an orgasm from 40th base....it sure was pleasureful...
by gretaaaaaaaaaa July 26, 2008
 
2.
having your roommate walk in while jacking off to a picture of his mom, and then joining....similtaniously sticking a frozen banana up a monkeys twat.
guy one: dude awesomest thing happened last night!!!!!!
guy two: what?!
guy one: i got some serious monkey action.
guy two: no way! 40th base?!?!? well done dude!

by dangletsbang August 01, 2008
 
4.
making out with somebody's 2cm flesh tunnel.
dan - duuuuuuuuude i just got to 40th base with ginny.
random stranger - you mean you kissed her 2cm stretched ear! aweeeeesome!
by gintingz July 28, 2008