Cream of the crop, of extraordinary quality.
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").
Man, this Hennessey is top-shelf.
192👍 88👎