In British slang, a tablet of amphetamine, or a benefit/advantage; also a nickname for Benjamin.
As a drug term, from 'benzedrine' (a brand of amphetamine inhaler used in the 1930s-40s). The term emerged in 1960s British drug culture. As a benefit, it's short for 'benefit.'
The word 'benny' shows how slang evolves from brand names—millions of people used Benzedrine inhalers that were legally sold to soldiers to stay awake in WWII, and the brand name got shortened and shifted into underground slang decades later.
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