People who are addicted to drugs, or colloquially, people who are obsessed with something and consume it excessively.
From 'junky,' which originally came from slang for heroin addicts (heroin was called 'junk'), then generalized to describe any addict. The term became widespread in the early-to-mid 20th century.
We now use 'junkie' for anything addictive—'information junkies,' 'exercise junkies'—and neuroscience shows that these aren't just metaphors; our brains release the same reward chemicals for different addictions, so the word accidentally describes something medically real.
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