Green’s Dictionary of Slang

(US) a person, a fellow, usu. foolish.

1921		T. Thursday ‘Hail the Professor’ in Top-Notch 1 Sept. 🌐 The only guaranteed wise cracker on this earth [...] is the gent that plays the part of the boob. It’s hard for that yamneck to loose [sic].
1939		J. Cary Mister Johnson (1952) 81: Oh my God, my dear bloody yamhead, what the father and mother of hell is seven and twopence from fifteen and a tanner?
1946		T. Thursday ‘Raw, Medium, and Well Done’ in Blue Ribbon Western June 🌐 These Western bean-heads are sick of looking at [...] Bearded Ladies, and Snake Charmers. This Flutterbutt yamneck will lay ’em in the cactus.
1947		T. Thursday ‘Sing Sing Sweeney’ in Crack Detective Jan. 🌐 So this yamhead liked to help folks out, huh?