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That's G, is basically like saying "Thats cool" or "That's awesome". People who use his phrase too much can get pretty annoying, but those who only drop it once and a while and at the right time, are pretty awesome. Some people drop the "that's" and just say "G", kinda like saying "word", but better!
"Hey, guess what i just won $500 dollars!"
"Dude, that's G!"

"Me and Natalie are going out tomorrow night! man is she hot!"
by Aliegurl March 05, 2010