A cainer is a british slang term for a person who likes to cain (take drugs, smoke weed, drink alchohol)
The word cain can mean to indulge in something to a rather extreme extent. This particularly relates to the 'caining' of drugs, alchohol and marijuana.
"That fuck heads a right cainer"
"My mate Daveo cained all of the coke"
"Alright cainer, shall we get hammered tonight?"
"That greeny lads a right cainer."