well, its not a knock on cos ball hasn't hit ground or another player. So it's just a fumble (aka an unclean catch).
Hmm... We could be back to the interpretation of the Boolean logic in this definition
[LAWS]Knock-on: When a player loses possession of the ball and it goes forward, or when a player hits the ball forward with the hand or arm, or when the ball hits the hand or arm and goes forward, and the ball touches the ground or another player before the original player can catch it. [/LAWS]
One could parse that many ways.
1) When a player loses possession of the ball and it goes forward,
OR when a player hits the ball forward with the hand or arm,
OR [ when the ball hits the hand or arm and goes forward, AND the ball touches the ground or another player before the original player can catch it. ]
or...
2) [ When a player loses possession of the ball and it goes forward,
OR when a player hits the ball forward with the hand or arm,
OR when the ball hits the hand or arm and goes forward, ]
AND
[ the ball touches the ground or another player before the original player can catch it. ]
I have seen both interpretations but (2) is the one I use.