The leftover spillage from errant shot pours which funnels into the rubber barmats which bartenders
typically divvy out to least-favorite patrons at
the end of
the night - "Hey, this one's on me!"
A jerk walks into a bar (lousy tipper or loudmouth prick)
and the bartenders start "missing" a few pours,
especially with more exotic liquors (and lots of them).
Eventually, there's enough in the barmat..."I think THAT guy deserves a barmat shot...this one's on me!"