Definition of "mumbler"

Attending Bradly for company when he went abroad with his painting traps failed to account for itself under the heading of a diversion. Old Mudgy surpassed himself as a scowler and mumbler when engaged in the practice of his art.

1938, Norman Lindsay, Age of Consent, 1st Australian edition, Sydney, N.S.W.: Ure Smith, published 1962, page 91