scum·ble / ˈskəmbəl/ Art • v. [tr.] modify (a painting or color) by applying a very thin coat of opaque paint to give a softer or duller effect. ∎  modify (a drawing) in a similar way with light shading in pencil or charcoal.• n. a thin, opaque coat of paint or layer of shading applied to give a softer or duller effect. ∎  the effect produced by adding such a coat or layer.

scumble spread colour over a picture to soften hard lines, etc. XVIII. poss. f. SCUM vb. + -LE2.