The Grand Canyon refers to a girl who is extremely blown out. The relevence of this term is that, just like the real Grand Canyon, the chick's vagina is forever stretched for for a huge distance. Girls with Grand Canyon twats should be avoided, as they have seen more than their fair share of dicks and probably contain numerous STDs.

Big expandable pussy that can fit numerous dicks inside.
Stretched pussy

A steep-sided gorge carved by the Colorado River in the State of Arizona.

The grand Canyon is when you snort a line of coke starting from the clitoris all the way to the anus of a stripper

The area between a Male's asshole and his balls

While having sex with a girl, grab a rolling pin or other large shaped object and ream her twat. once you pull the object out and her gash is gaping, yell into her twat lips "Hello" and wait for the echo.

The act of spreading a girl's ass cheeks or vaginal lips, then ejaculating at the top and watching the cum drip down the crack, mimicking the river that flows in the Grand Canyon.