This achievement is for obtaining the Art Dealer's safe combination, but breaking into it before handing it over to SlackJaw.
During the 3rd Mission 'House of Pleasure', you are given the opportunity to complete a few side quests for SlackJaw first.
*** Spoiler - click to reveal ***The first one involves breaking into Dr. Galvani's house to find out what happened to his informant. Do this and return to SlackJaw for the next part of the quest.
For the second quest, SlackJaw sends you to the Golden Cat to get the safe combination from the Art Dealer in exchange for his helping to take out the Pendleton twins. You will have to "torture" him a bit to get it. Do so and head to his house. Break into the safe and steal the contents.
Return to SlackJaw a second time and give him the combination to the safe.
The achievement will unlock right after this.