A similar example can be found in Billy Joel's song "Zanzibar" in which he compares himself to Pete Rose and sings the lines "Me, I'm trying just to get to second base and I'd steal it if she only gave the sign.She's gonna give the go ahead, the inning isn't over yet for me." David Letterman chronicled many of these in his "Top Ten Baseball Euphemisms for Sex" — a recurring theme on the Top Ten Lists featured on the late-night talk shows he hosted before retiring in May, 2015.
He explains this by saying "That's two home runs, and then she gives me back a triple." Home run is defined: Thank you for your interest in this question.
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Talking about fifth base when there's really only four bases reminds me of the expression Up To Eleven.
If you want to downvote me for using Urban Dictionary, that's fine. First base When you get to first base, you have been lucky enough to have been kissed.
It also includes other forms of petting, touching and groping.