Scumbling is much more than a fun word to say, it is a useful technique in both drawing and painting.
It refers to a sort of dry brush or scribble technique that aids in natural looking shading.
Scumbling is best done with a firm or coarse brush. The paint is scrubbed in, often over another layer in a different color.
It might seem like a new-fangled thing but in fact, Rembrandt often employed this technique.
Scumbling creates fantastic atmosphere, texture and light variations.
If you are interested this is a video illustrating the technique.