WordPress at a glance
action is not described

delete_(meta_type)meta action-hook . WP 3.4.0

Fires immediately before deleting post or comment metadata of a specific type.

The dynamic portion of the hook, $meta_type, refers to the meta object type (post or comment).

Usage

add_action( 'delete_(meta_type)meta', 'action_function_name_5944' );
function action_function_name_5944( $meta_id ){
	// action...
}
$meta_id(int)
ID of the metadata entry to delete.

Where the hook is called

delete_metadata_by_mid()
wp-includes/meta.php 800
do_action( "delete_{$meta_type}meta", $meta_id );

Where the hook is used (in WP core)

Does not used.