Green’s Dictionary of Slang

an unpleasant person.

1905		C. Chesnutt Colonel’s Dream 207: Who is this mucker, anyway?
1923		O.O. McIntyre New York Day by Day 23 Sept. [synd. col.] My chauffeur [...] wants two nights off a week and extra pay for Sunday. Can you feature a mucker like that?
1926		Wood & Goddard Dict. Amer. Sl.
1938		L. Ortzen Down Donkey Row 71: The silly old mucker.
1944		J. Cary Horse’s Mouth (1948) 344: Take it away, Madgie, you mucker.
1965		D. Martin Hero of Too 324: Well laugh, then, you muckers.