The Slang and Euphemism Dictionary, Richard Spears, says that "marijuana" is from the US underworld and drug culture of the 1900s, and gives about 200 synonyms.
Origins A Short Etymological Dictionary of Modern English, Eric Partridge, offers:
marijuana or marihuana: ASp mariguana, var marihauna: o.o.o. For mariguana, a "vulgar Mexican name for the common hemp" -- whence its A use -- Santamaria tentatively suggests origin in Mariguana, one of the islands forming the Bahamas.
FWIW - even the spelling was unsettled in Michigan courts as recently as 30 years ago. We've landed on marijuana.
