Green’s Dictionary of Slang

split-arse v.

also split-ass
[SE split + arse n. (1); the movement of one’s legs and buttocks]

1. to move quickly.

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[UK] ‘Omer Drome’ in C.H. Ward-Jackson Airman’s Song Book (1945) 45: So won’t you split-arse back / Along the track / To my dear old Omer Town?
[UK] ‘My Old Yellow Jacket’ in C.H. Ward-Jackson Airman’s Song Book (1945) 47: He’s returning our fire, / I must split-arse and stunt round him.

2. to act recklessly.

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[UK]A.S.G. Lee letter in No Parachute (1968) 24 May 18: You can do what you like with your own bus, if you want to loosen it up with show-off splitassing, that’s you own affair.