The Grimy key is an item needed to open a small locked room in the main floor of Draynor Manor get the rubber tube (one of the three quest items needed to finish the quest Ernest the Chicken).
The key is found in the compost heap on the western side of Draynor Manor, within the fence of the property. Once the player is done with the quest, this item can no longer be obtained.
Note: the name "smelly key" was the former name of this item. This item was then known as "key", and now named as "grimy key".