"Ah, ah, ah, you didn't say the magic word!"
Join any multiplayer game in the Server Browser.
Once you are in a game and on a team, press T or Y to bring up the chat.
Type the word "please" into chat during the game, hit Enter, and the secret achievement will unlock.
NOTE: This doesn't seem to unlock if playing solo, either through the Solo option or creating a game yourself through the Multiplayer/Host Game option.