meaning of "numbling"

This seems simply to be a misreading of handwriting. Every word has a meaning. Even scumbling is not a nonsense word.

The first letter of both those words could be an r which would make rambling preceded by rumbling. In fact, to make the first three letters on that line num seems not to be possible as there are not enough strokes for it.

Rumbling seems far more likely. It doesn't appear anywhere else in that paragraph, and provides a neat bridge between stumbling (and all the other -umbling words) and rambling.

(The extract of the handwritten letter comes from the OP reference on lettersofnote.com)