When you take something that exists on the internet, print it out using a 3D printer, and then, using copious amounts of lubrication, insert it in your rectum.
Verb: to Derf is to purposfully send someone the wrong way, or to trick someone into going the wrong way. You have to strike it with such confidence and not to stutter that they do not second guess you and still believe they are going the right way. Even when they know they have gone the wrong way, they blame or doubt themselves.