Green’s Dictionary of Slang

umpchay n.

a fool.

1900–10		Stephens & O’Brien Materials for a Dict. of Aus. Sl. [unpub. ms.] 140: ‘Screw the umpcha [sic] on the left chalk,’ look at the chump on your left hand.
1910		Sun. Times (Perth) 6 Feb. 2nd sect. 1/1: They Say [...] That when two little boys demanded their [surfboards’] return the huge bully talked of ducking them. That if the threat had been executed the colossal um-chaw would have gone in first.
1929		Hostetter & Beesley It’s a Racket! 241: umpchay — A foolish or stupid person.
1933		W.R. Burnett Dark Hazard (1934) 9: What does this umpchay know about it?