Bottle-holder Pocket 

"You mean a cup-holder pocket?"
"Nah, dude! Cups could spill!"

{bot-l-hohl-der pok-it} noun, plural Bottle-holder Pockets {-its}

The netting on the outside of a backpack (bag, fanny pack, etc.) that is made specifically for holding water bottles.
Not to be confused with a "front-zipper pocket."

Slate: Hey man! Could you get my bottle of jank (jenkem) out of my backpack?
Ray man: Sure, dude.. Wait I can't find it. Are you sure it's in here?
Slate: Yeah, chief. It's in the Bottle-holder pocket.
Ray man: Oh. Jeez. Found it. Here you go.
Slate: Nah, nig. You take browns.

Bottle-holder Pocket by refinedhippie November 2, 2012