The word caining comes from a primitive method of stealing from shops especially ones that sell clothes. The process of caining involves
pushing a long bamboo cain through the
letter box of the shop you are trying to rob and hooking items (IE clothes)
off the rails and by tilting the cain upwards the items slide to the mouth of the letter box where they are easily pulled though. The phrase became associated with the UK drug culture in the mid 90's as caining was an easy way of obtaining money to buy drugs.
The word 'cained' refers to being off your face with the money you made from caining.As the price of drugs like Ecstasy dropped as much as 1/20th of the original
street price the need for large amounts of money to buy drugs had decreased and the word is now used as a general term that refers to any state of altered consciousness gained through the use of illegal drugs.
If someone is said to be cained it usually means they have taken so much mind altering substances that they are physically incapable of taking any more.