A state of mind, achieved by taking drugs to the point that you are impaired by them. As appose to the euphoric feeling you get, the high.
It often refers to marijuana intoxication, however could also be used to describe any intoxicated state. (eg. while on mushrooms, alcohol etc ... ). Other drugs have don't have a stoning affect (or one at used dosages) such as MDMA, which can be very clear headed.
Caution it is still not recommended you drive while under the influence of any drugs, stoned or not.
This is gooooood weeeeed..... Im very stoned.......
Mushrooms got me so stoned, I couldn't roll a joint.
"And the waitress is practising politics
As the businessmen slowly get stoned"
-Billy Joel, Piano Man lyrics