MWCD comments: Some commentators insist that use of masterful should be limited to sense 1 in order to preserve a distinction between it and masterly. The distinction is a modern one, excogitated by a 20th century pundit in disregard of the history of the word. Both words developed in a parallel manner but the earlier sense of masterly, equivalent to masterful 1, dropped out of use. Since masterly had but one sense, the pundit opined that it would be tidy if masterful were likewise limited to one sense and he forthwith condemned use of masterful 2 as an error. Sense 2 of masterful, which is slightly older than the sense of masterly intended to replace it, has continued in reputable use all along; it cannot rationally be called an error. [see here for senses 1 and 2]