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When you mouth is so dry, with just a tad of saliva covering you sandy dry tongue....resembling a sand bar with only a tad bit of water over it....most commonly encountered while high, a variation of cotton mouth.
Bud: Hey man this is some dank bud.

Cotton: Yeah but it's giving me the worst cotton mouth.

Sandy: Yeah you must have "Sandbar Mouth".....ya

know...when your mouth is so dry it resembles a sandbar?
by Eli Packet January 29, 2011