_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_2666' );
function action_function_name_2666(){
	// action...
}

Where the hook is called

do_action( '_admin_menu' );

Where the hook is used (in WP core)

add_action( '_admin_menu', 'wp_widgets_add_menu' );