Lucky Guess is a secret achievement/trophy in Deus Ex: Human Revolution. It is earned by entering the code 0000 to the bomb being used by Jacob White during the side quest, "Smash the State", instead of hacking it. Alternatively, any means of disabling it except hacking will also earn this achievement.
During the side-quest "Smash the State" (which you get after learning Sandoval's location either by convincing Taggart or by hacking his computer), you simply have to guess the code (which is 0000) to disarm the bomb instead of hacking it. You can also unlock this achievement by using an Automatic Unlocking Device to defuse the bomb.