WordPress at a glance
action is not described

_admin_menu action-hook . WP 2.2.0

Fires before the administration menu loads in the admin.

The hook fires before menus and sub-menus are removed based on user privileges.

Usage

add_action( '_admin_menu', 'action_function_name_2030' );
function action_function_name_2030(){
	// action...
}

Where the hook is called

In file: /wp-admin/includes/menu.php
_admin_menu
wp-admin/includes/menu.php 41
do_action( '_admin_menu' );

Where the hook is used (in WP core)

wp-includes/functions.php 3721
add_action( '_admin_menu', 'wp_widgets_add_menu' );