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1.
A device used to smoke marijuana, usualluy fashioned out of a steam roller bong and a hose connecting it to a piece that covers your nose and mouth. A grass mask will completely fill your lungs through your nose as well as your mouth
"That grass mask kicked my ass last night."
by Old man withers August 03, 2005
 
2.
A gas mask fitted with a bong instead of a filter, the purpose being to smoke weed while wearing this mask.
We wandered around until we found a grassmask for sale. Zach and I bought and went home and got blazed as hell!
by Vindikation April 11, 2007