Green’s Dictionary of Slang

1. a grimace.

1892

W. Norr Stories of Chinatown 21: I’d like ter punch the monkey face off’m.

2. a stupid, ugly person, also as a term of address.

1895		Cornishman 22 Aug. 6/3: ‘You thought it insulting to be called “Monkey-face”,’asked a solictor [...] ‘I did,’ replied the plaintiff.
1896		F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 238: Grover Cleveland is a monkey face.
1925		Dos Passos Manhattan Transfer 222: We are going to vindicate the honor of poor old monkeyface.
1936		C. Sandburg People, Yes 130: Where did ja cop dat monkeyface.
1972		T.C. Bambara Gorilla, My Love 82: What are you doing, monkeyface?
1977		A. Bleasdale No More Sitting on the Old School Bench (1979) 65: fourth boy: Slime first negro: Monkey face.