Intoxicated by drugs, particularly marijuana, resulting in an altered state of consciousness. Originally, it meant pelted with stones as punishment.
The punishment meaning comes directly from throwing stones. The drug-related meaning emerged in 1960s American slang, possibly from the sense of being 'stone-still' or immobilized, or from earlier 'stoned drunk'.
The evolution from 'pelted with rocks' to 'high on drugs' shows how slang transforms through metaphor - both meanings suggest being overwhelmed or altered by external forces, just very differently!
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