Green’s Dictionary of Slang

1. esp. of a baby girl, female.

1954		in Randolph & Legman Ozark Folksongs and Folklore (1992) I 379: A British Cockney artist announcing [...] the birth of a disappointing daughter instead of a hoped-for son, said disgustedly: ‘Just another split-arsed bleeder!’.
1984		Partridge DSUE (8th edn) 1129/1: split-arsed one A female (esp. baby) [...] late C.19–20.

2. of a man, cowardly, effeminate.

1981

A. Weller Day of the Dog 88: Ya skinny split-arsed yellah bastard.