In a recent post about his article on the origin of the pseudonym "Mark
Twain," Kevin MacDonnell writes:

"The bar tab story (with an entirely different meaning) is based on
patently false newspaper accounts by people who did not know or ever meet
Twain."

While researching for my book, I ran across a report that the bar tab story
was a deliberate yarn told by Alf Doten, who did know SLC, as a joke on
Sam:

"Doten delighted in pretending to their new acquaintances that it [nom de
plume] originated from Clemens using the expression in the booming days of
Virginia City on such occasions as he found it convenient to 'stand off" a
friendly bartender for drinks for Doten and himself."

So that "origin" story is clearly not to be believed, but no doubt was
circulated.