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Meaning Hashish, from cockney rhyme slang. From country/western performer Johnny Cash, which rhymes with hash, a popular colloquialism for hashish.
What are we going to do with all the cash? Smoke Johnny! And throw a bash!
by Robert L. Orvin June 22, 2006
comes from the cockney rhyming slang of High House
Hash rhymes with Cash, thus Johnny Cash was shortened to simply Johnny

History/Origin: In the Tenacious D song "Kyle Quit the Band," the lyric is "What we gonna do with all the cash? Smoke Hash!"
"Let's fuckin' smoke some johny"

"Fuckin mix the johnny and the dirty in that there spliff, son"
by pig on dirty August 08, 2006