We ran the definitive answer to this question on 10/21/06, so we figured enough time had elapsed for us to re-run it. Besides, it's one of our more colorful and in-depth efforts:
Oh my! We're guessing you had no idea what a can of worms you opened when you innocently lobbed this hot potato our way. Still, we're not afraid of a few worms so, to push our mixed metaphors just a little further, we're going to take this bull by the horns.
First, for those who don't know, the act of being 86'd from a casino equates with being read the Trespass Act, meaning that you’re barred from the property in question and risk being arrested should you attempt to enter it thereafter. But where did the term come from?
On the surface, the number 86 seems innocent enough. In fact, in mathematical parlance it's among the 15% or so "happy numbers" (we'd hate to digress further, but you can Google that if you don't know and do care what it means). It's also, among other things, the atomic number of radon, the number of the French department of Vienne, the designation of a highway in Idaho (and others in Pennsylvania and New York), and the international dialing code for China. None of these has any bearing on the number's use in reference to being thrown out of a casino, however, although the fact that 86 is also a common proof for scotch whiskies might. But more on that later...
There are, in fact, many theories for the origin of the term "86'd," none of which has been or can be proven definitively. One thing they all have in common, however, is that they have nothing to do with the world of gambling, so this obviously represents a later application of an already established phrase. Many versions are related to restaurants and bars and our gut feeling is that this general area is the most likely source, but we'll list all the explanations we've come across, in an approximate reverse order of likelihood:
You’re probably sorry you asked, and we’re sorry that we can’t give you a definite answer, but the above should certainly be some food for thought.