Noun. fifth base. (colloquial) anal sex.

What does 5th base mean?

She said second base was copping a feel, third base was hands (or more?) in the pants, and a home run was sex. Well, an inside source just sent me a little tip about another base… (Shy readers, look away!) The fifth base is—you guessed it—going through the back door. Eeps!

How many types of bases are there?

On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.