The player can earn this achievement by typing "mustache" or "moustache" while the game is being played. The player can activate this code at any time in the game. There are three types of mustaches as seen below.
If you do this, in Steam you will unlock the achievement Ask Me About Mustache Mode. In the GOTY edition, this was simplified to Mustache Mode. These achievements are regarded as the easiest achievement in the game, since they require no talent.