What is Greenhouse?
1.
To go get stoned/high
"Hey lets go greenhouse" meaning "lets go smoke weed"
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2.
To smoke weed inside a closed environment, such as a car with the windows rolled up. Inhaling the second-hand smoke increases the effect.
We sat in the car and greenhoused a couple of joints.
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3.
RAF wartime slang for Cockpit cover.
"Couldn't get the greenhouse open"
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