From where comes the restaurant term "86" which stands for out of stock?

Answer: Its original origin can't precisely be identified.

The term eighty-six is restaurant/bar slang for an item that is out of stock or a customer that is to be denied service. The origin is obscure. The earliest clear reference is to the February 1936 issue of American Speech; it was undoubtedly in existence before that. Lighter cites a 1926-35 comedy where a waiter gives his number as eighty-six.

The OED2 postulates that it may be rhyming slang for nix, and most authorities tend to go with this explanation although there is
no strong evidence to support it. It is plausible as restaurant crews frequently employ codes such as this.

American Heritage suggests that it may derive from Chumley's Bar in Greenwich Village, which is located at 86 Bedford Street.

There are other explanations at these sites as well.

http://www.urbandictionary.com/define.php?term=86

Indeed, there are a lot of possibilities for the origin.

1933:

The most widely accepted theory of the term's origin states it derives from a code supposedly used in some restaurants[5] in the 1930s, wherein 86 was a shortform among restaurant workers for 'We're all out of it.' Snippets of said code were published in newsman Walter Winchell's column in 1933, where it was presented as part of a "glossary of soda-fountain lingo."


Wiki suggests any of the following pre-1950 things could also possibly be the origin:

- United States Navy decommissioning
- 86 Bedford Street police story
- From a typer of camera filter
- From Electrician slang

https://en.wikipedia.org/wiki/86_(term)