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.
Justin decided that he was going to have sex with his new girlfriend Megan, but when he took off her clothes he wisely decided to back out, due to her Grand Canyon vagina.
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.