Bill,

In America to get hit in the chops means to get hit in the mouth. Chops started out as an expression for horn players that needed to gain or keep their embashure, the muscles around their mouth. These muscles need to be in good shape to hit the notes. With practice these muscles get strong and stay in shape. The by product of all this practice is not only great muscles of the embashure but proficiency in playing.

The expression later evolved into meaning good proficiency on any instrument.

Tom