People who smoke marijuana as a regular habit, or in historical contexts, people who throw stones.
From 'stone' plus '-er' (one who does something); modern slang usage for cannabis users emerged in the 1960s-70s.
The term 'stoner' was basically invented by counterculture in the 1960s when marijuana became associated with rock music—but the original meaning was just someone who threw stones, showing how slang completely changes word meanings!
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