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!

6900th base by Fyckboi72 August 7, 2017

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fourth-base shortcut

Refers to the act of bypassing second and third base, but instead proceeding directly from first to fourth base; i.e., having sex with a girl right away, without bothering to tediously wade through the whole hugging/kissing/touchy-feely foreplay-crap in between.

The simplest and most effective/successful fourth-base shortcut procedure, in two easy steps:
(1) Find a hot chick and offer her a free foot-massage (first base).
(2) When the cutie obligingly plops herself down and gives you her feet, take one foot in each hand and start rubbing them, but also gradually raise them up high till her legs are back over her head and her butt is lifted off the ground, allowing you to slip off whatever lower garment(s) she's wearing. If she'd made no objections by that time, then the rest is easy; just kick off your own lower garments and enjoy (fourth base).

fourth-base shortcut by QuacksO October 8, 2017

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6th base

6th base is when you stick your penis down a woman’s ear.

I went to 6th base with Linda last night.

6th base by Barboner September 23, 2017

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