"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.
one can be said to be "lickin' walls" when they find themselves in a desperate situation where they want to smoke but have no weed and therefore must lick the walls to get the TCH residue out of the walls of the room that is normally smoked in.
You can not actually get TCH from licking the walls and this is just meant to be used to express the desperate situation you are in.
I've been dry for days son. I'm lickin' walls!