V.M. Yeates spots a German aeroplane:
It was one of the new DFWs – a nice-looking, very splitarse bus.
‘Splitarsing’ is a frequent word in Winged Victory It appears in Arnall’s Portrait of an Airman, too. The context makes it clear that it means ‘making a fast manoeuvre’, and the Oxford English Dictionary gives us :
split-arse v. Air Force slang. intr. to make a sudden turn in an aircraft; to perform stunt flying.
The OED quotes Nettleingham in Tommy’s Tunes (1917) ‘The expression ‘a splitass merchant’ is applied indiscriminately to a reckless individual or to a really good flyer capable of executing stunts with a modicum of safety.’
On the term’s origin, the OED is disappointing:
Etymology: < split adj. + arse n.
A “Split-Arsed pilot” or to refer to a “split-arse turn” was a reference from way back to the first combats and aerobatics when tight turns when flying “By the seat of your pants” was the norm. This was also known as Splitarsing around in combat or in Aerobatics. This is due to the effect on your errr, base(?), you know the word, Arse, as it slides around on the seat due to sideways pressure.
Well, maybe, but a 2006 edition of partridge’s Dictionary of Slang is more helpful, and takes the term further back. It quotes Col Archie White, V.C., who writes: ‘A cavalry term before the RAF came up. A horse, at its fullest gallop and seen from behind, visibly splays its legs.’
This would makes sense as an origin, since many WW1 pilots came from the cavalry, it being believed that men who had the touch with a horse would also have it with an aircraft. (Motorcyclists were also recruited).
Yet the OED refers to an earlier usage, from 1903:
Split-arse mechanic,..a harlot.
and ‘splitarse’ or just ‘split’ is sometimes found later as a military slang term for a woman.
The origins of slang terms are never simple.