"81" is a numerical code in bar calls, and is shorthand code for top-ups in water.
If you hear this at a venue, you'd most likely hear "81s on (table number)", meaning whatever table they call needs a top up on that
fresh water.
"Round of 81s" means every table where the pitcher is empty needs that thing filled with H2O.
While 81 might seem less
efficient to say, due to 81 having 3
syllables if said as "eighty-one", which is one more syllable than the actual word "water", it's still all part of the work to make things discreet.
Bartender 1 walks about, passing by every table. 8, 9, 10, 11…the water pitcher's empty on this one…
Bartender 1 81s on 12.
One of the other bartenders hears the request and fills the pitcher with water, so
the next customers to sit on table 12 have
something to quench their thirst.