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!"