The line comes from Team Fortress 2, Valve's class based shooter released in 2007, where the Sniper is a professional assassin character. It became widely quoted after the Meet the Sniper video, where he explains his code while working. The phrase spread through TF2 frag videos, forums and meme edits, and players adopted it as shorthand for doing a difficult job properly. It remains popular because the wording is instantly recognisable to shooter fans and easy to apply outside the game.