A slang term for wasting time. Originated from the game Modern Warfare, in which you can blow out windows and, if there are small pieces of glass still in the window frame, you can use your knife to slice out the little pieces. The activity does not have any point and is useless in the game, therefore it wastes time.
Slicing the devil’s neck means deliberately going out of your way to start trouble/problems just so you can face some form of consequence/danger to yourself.