'Yay' comes from a Spanish word for cocaine: 'yeyo' (also referred to on this site and the film Scarface). 'K' is slang for ketamine and the phrase rhymes with the dangerous weapon 'AK47' - referring to the drugs dangerous and uncertain nature.
The term was first used and created on 05/07/09 in Hyde Park, London, England by a guy 'Mustang D' who incidentally & unfortunately ended up in a festival medical tent due to the effects of a mixture of Yay-K47 and various other narcotics (illustrating the potentially dangerous nature of the drug).
In a very short space of time the term has since become an extremely popular substitute for the previous slang word of the same meaning: 'CK1' (cocaine-ketamine) - due to it's ingenuity. It is currently in use around the South of England but is rapidly spreading around the world due to the extensive use of the Internet and social networking websites.
Official spelling is 'Yay-K47' but 'YayK47', 'Yey-K-47' have also been used.
Buying Yay-K47 from a stranger can be dangerous as it is impossible to truely know the ratio of the mix and whether or not other narcotics are present. Mixing narcotics or any drugs can be extremely dangerous.
"wanna get off ya face? get sum Yay-K47, it'll blow ya away!"