Knock Knock is a secret achievement/trophy in the System Rift expansion for Deus Ex: Mankind Divided. It is awarded for talking to Slaw in the Palisade Blade, and getting him to open one of the security doors.
Slaw can be found on level 2 of the main section of the Blade. He is located in a maintenance area on the west side of the building, and is standing in front of a locked security door. By talking to him, he will open the door for you, so that you can access level 3 more easily without being seen by the Tarvos Security Services guards.
Slaw will ask you to knock him out in exchange for unlocking the door so it looks like he "put up a fight". However it is optional to do so and you will be awarded the achievement regardless.