A bastardisation of the phrase "mothering the brushes" meaning to gain sexual pleasures from an ordinary household object the same way a dildo works. There is now a small company in south-east England manufacturing mumbrushes for sale.
Also: mumming the brush. To use a mumbrush.
"Did you see Coling go with that mumbrush!?"
"I mummed the brush all fucking day and nothing fucking happened, I'm complaining to my lawyer!"