6900th base

The act of sexual intercourse in your grandmother's bed. Little known fact, it's just weird unless it's also in a public area such as the Leaning Tower of Piza, Empire State Building, or some other tourist filled shithole.

Guy 1: I got to 6900th base last night!
Guy 2: No way! Where did you put your grandma's bed?
Guy 1: The Eiffel Tower!

Guy 3: That's so romantic Bro!