[pwg]

Another old example.

WordStar: Used "X" to Exit to system in its main menu (https://www.flickr.com/photos/markgregory/6946218793/?rb=1) - I do not know the revision shown in the screen shot.

According to Wikipedia (http://en.wikipedia.org/wiki/WordStar) WordStar was released in 1978. Which moves the date back to at least 1978 to use X for exit.

However, there is possibly a very simple explanation that the blog posting overlooked. In text menu's, such as WordStar's, which were quite common for a lot of software from that era, using the word "Exit" to mean "leave this program/application" was also common. When one goes looking for a single character memonic for "Exit" to build in as a keystroke to activate the "Exit" command from the menu, one has four choices: [e] [x] [i] [t]

Since [x] is an uncommon letter, while e, i, t, are more common, and therefore more likely to be used for triggering other commands in the menu(s), choosing [x] to mean exit meant that the same character could likely be used as a universal "leave this menu" command key across all the menus.

Which would then lead to the common _F_ile->E_x_it command accelerators in drop down style menus (whether in a GUI or in a text menuing system). [x] was unlikely to have been used for the keyboard accelerator for other entries in the "file" menu, so picking e[x]it was a safe choice.

It is not a far reach from _F_ile->E_x_it using [x] as its accelerator key to labeling the title bar button that performs the same function with an X as well, to take advantage of whatever familiarity users might have with the drop down menu accelerators

[crb3]

In WordStar it's a little more nuanced than that.

First, it's properly ^K X, as the ^K prefix subcommands block/file actions, as written by Rob Barnaby into all the WordStar versions starting with CP/M.

Second, ^KX as 'exit' means to save the latest revisions out to file before quitting, while ^KQ, 'quit', means to abandon the revisions. You might get a confirmation dialog and a chance to change your mind before you're dumped back to the commandline.

Current-convention iconic close-window behavior more closely emulates the latter.

[pwg]

Correct on the commands while editing a document, but I was specifically referring to the WordStar start screen before one begins editing a document (check the flickr link) where it has "X EXIT to system" (the all caps is also in the screen shot).

On WordStar 7.0a for dos, the main start screen menu selection is "X exit WordStar".

[malkia]

I think Turbo Pascal worked that way too

[hammock]

Not to mention the letter X sounds like the word "exit," or "ex it." In this way using X as a keyboard command to represent exit might be distinct from the way OP posits X to represent batsu (false, bad, wrong or attack).

[hyp0]

though X already means false or bad in western culture, the opposite of a tick (✓). there's also "ex-partner" (my ex) etc. in a gui, it's an ex-window.

[Al-Khwarizmi]

In Ultima IV (1985) the spell for exiting a dungeon was the X-it spell (activated with the letter X).

[autokad]

this helps prove your point. This is from Qbasic that ran in DOS. I use to run this on my windows 3.1 all the time:

http://www.mysundial.ca/tsp/images/qbasic_screen_2.gif

[DanBC]

Interesting that that screenshot shows other windowing decorations, but that "close" is hidden in a menu option.

[autokad]

ignore the outer part, thats from a newer version of windows. i remember using the short cuts for x to exit so i searched for a screen shot that had the file menu open

[ChrisClark]

Alt-F-S Alt-F-X

Used many times.

[DanBC]

The inner part shows scrollbars. MS Edit has alerts and buttons and scrollbars.