Uncertain origin, perhaps from scum.
opaque kind of glaze
- to apply an opaque glaze to an area of a painting to make it softer or duller
- 2000: The moon was brilliant, the path a track of scumbled footprints in the snow, the air cutting and cold. — Philip Pullman, The Amber Spyglass
- For usage examples of this term, see Citations:scumble.