This is a gem. The hidden language of bars. Completely beautiful nonsense. Intriguing little microclimates of language — some which seem to exist between bars, and some within.
A specially prepared drink that is sealed (say, with plastic wrap or a rubber glove) and dispatched as a gift to a nearby bar. Of dubious legality, BOOMERANGS are a way of ‘having a drink’ with industry friends during work. BOOMERANGS are often shuttled from bar to bar by regulars, who are thereby identified as guests of quality.
[I] One who sneaks out, leaving his friends to pay.  A cool and composed drinker.
Wealthy client, not spending.