2) can also be used to describe anything that is made up of prime ingredients: marajuana, pussy, hardware, anything really.
2) A: This is the best weed I have ever smoked!
B: I was saving it for a rainy day, straight off the top shelf.
The phrase is derived, not from the position of premium bottles in liquor stores, but their position behind bars. (as opposed to "well" liquors, which are stored in the bartender's speedrack well, and "call" liquors, which are popular but not displayed (and which must be "called").