Scumbling is a painting technique which I first heard used to describe the technique of achieving a wood grain effect on a narrow boat door.
Pit shirt is a shirt worn to work in the pit, most likely an over shirt.
As for bumble, I was once very drunk and latterly very sick through drinking Scrumpy with a shot of gin in it, which the locals in Polperro assured me was the right way to drink it. I think they call it teasing the Grockle in that part of the country.