A feeling one gets when he or she feels really messed-up in a pleasurable way. A state of conscience one obtains through enjoying mind-altering substances.
Dude I am So top-shelf right now!
Two blue-footballs and I'm top-shelf most of the day!
What's your dosage for that top-shelf feeling?
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").