When you take either your left or right hand and extend all fingers, with your thumb extended along the hand, much like a karate chop and you point your "knife hand" at other people and tell them how fucked up they are. Using lots of expletive deleteds are exceptable. You can also throw knife hands, which mean you are point your knife hand furiously at an individual in hopes he or she will retaliate with physical force in which you will ankle pick them and throw knife hands into their throat.
Serves as a hand signal and way of pointing at things. A knifehand is done by fully extending and adjoining the thumb and fingers. If somebody points a knifehand in your direction, you're about to have a really bad day. It is also a strike in mcmap