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?