"Lickin' Walls" is a metaphor used by some stoners to denote the point where all possible routes of obtaining weed have been exhausted. Desperate measures such as cleaning out the grinder entirely and hitting all the resin possible out of the stoner's bowls have been taken. All that is left is to lick the walls and hope that somehow the weed that has seeped into them will get the stoner high. The more desperate counterpart to lickin' walls is burnin' hair.
Stoner A: Dude, you sure we have nothin left?
Stoner B: Nah trick, we're so out we might have to start lickin' walls.
Stoner A: Shit man, I hate getting splinters on my tongue.