1. silly behaviour, nonsense ...1918 obs. rare, originally and chiefly US usage
2. (often derogatory) an eastern or southeastern European, esp. of the working class; a Bohemian; a foreigner ...1924 Amer. dial.
3. a stupid, obnoxious, or worthless person; often used as a term of abuse or a contemptuous form of address ...1933
4. a person's buttocks; the bottom, the backside ...1941
apparently from the title of a comic song and the name of its rustic and unintelligent character, Bohunkus (1870s)
FIRST DOCUMENTED USE
1918 - see EXAMPLE below
"...Now look here! Any more shilly-shallying and bohunkus and I fire the both of you. See?..."
From: The Saturday Evening Post (Philadelphia)
14 Dec, 1918
in: Suffering Husbands, by Wallace Irwin