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.
by Tit For Twat January 18, 2007
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.
She was begging me to cum on her, so I spread those cheeks and gave her the Grand Canyon.
by recklessxx November 15, 2011