a Key can refer to one of 2 things. firstly, it can refer to a kilo of a drug
secondly, it can refer to the act of using a house key to take drugs, such as coke, when there is no other way to do so (no snorter, in a club etc).
Jake: i almost got caught with a key of coke in my car yester day
Dave: we didnt have any notes to make a snorter out of so we just did keys instead