This is only available on steam, requiring you to enter a “cheat code” to activate this. It does nothing, except turn on moustaches.
To earn this achievement, the player must simply type “mustache” or “moustache” while playing the game or in the main menu, which will make some types of zombies have mustaches. If the player types it correctly, the achievement will be earned.
Source : Link , Question Author : oliverthegreat , Answer Author : Ben