Green’s Dictionary of Slang

split-arse adj.

daring.

1927		(con. 1925) ‘J.H. Ross’ Mint (1955) 203: ‘More split-arse work tonight?’ It pleases them to imagine me wild on the road.
1929		E.S. McCartney ‘Sl. and Idioms of World War’ Papers Michigan Academy of Arts & Sciences 10 325: Split-ass merchant, an expression applied indiscriminately to a reckless individual, or to a really good flyer capable of executing stunts with a modicum of safety.
1946		G. Gibson Enemy Coast Ahead (1955) 20: He was slightly more split-arse than Hoppy, and flying to him was nothing unless it was dangerous.