What is the meaning of abbreviation ynaq in nethack?

Choosing a means no takebacks are allowed for this choice.

The explanation can be found in the game's code. For example, lines 544 through 548 of /win/tty/wintty.c (windows version, console menu) state:

    /* this prompt string ends in "[ynaq]?":
       y - game picks role,&c then asks player to confirm;
       n - player manually chooses via menu selections;
       a - like 'y', but skips confirmation and starts game;
       q - quit
     */

so it appears that choosing the a option does the same as choosing the y option (randomize your role), but does not allow you to confirm or cancel if you got a combination you don't want to play, instead it will immediately start the game.

Looking a bit further at line 950:

getconfirmation = (picksomething && pick4u != 'a' && !flags.randomall);

The boolean getconfirmation is always false if pick4u (containing the player's choice) is set to a. Thus the program will not run the code asking for confirmation, instead leaving the function and presumably going on to setup the game.

It's hard to find information on this, and I've often wondered it myself, but it seems to stand for "all". Even the Nethack Wiki doesn't define "ynaq".

In the case of item accumulation:

• Typing y picks up that item, and displays the next.
• Typing n displays the next item.
• Typing a picks up that item, and all remaining items of the named types. The program will not allow the player to pick up any item which is too heavy.
• Typing q aborts the command.

In the case of starting a new game, it would make all relevant choices for you so the game can begin in earnest.

Do note that when pressing q your character will be deleted. It's best to save instead using Shift+S.

References:

https://thekill08.wordpress.com/2008/03/15/hack-and-nethack-the-software-distillery/ ftp://ftp.cuhk.edu.hk/pub/oldies/apple2/unsorted/Hack.txt