1590s in a literal sense, agent noun from push (v.). Meaning "peddler of illegal drugs" (1935 in prison slang) is from the verb in the "promote" sense.
Slang definitions & phrases for pusher
pusher
noun
A narcotics peddler or distributor; candy man, connection: queen of the Broadway narcotics pushers(1935+ Narcotics)
A distributor or passer of counterfeit money; paperhanger(1940s+ Underworld)