split-arse mechanic, n. — Green’s Dictionary of Slang

split-arse mechanic n.

[split arse n. (1) + SE mechanic; note RAF jargon split-arse merchant, a reckless, showy or daring airman; split-arse cap, the Royal Flying Corps cap, similar to a Glengarry]

1. a prostitute.

1890–1904

Farmer & Henley Sl. and Its Analogues.

2. a woman.

1984

(con. WW2) T. Jones Heart of Oak [ebook] Men who were reporting from their second or third sinking in the past eighteen months were [...] made to stand silently in line, forbidden to smoke by a big notice in six-inch letters on the yellow-painted wall, while split-ass mechanics [...] puffed away at their Black Cat corked tips and re-touched their lipstick.

In phrases

take on a split-arse(d) mechanic (v.)

to have sexual intercourse.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 113: Étrangler. To copulate.; ‘to take on a split-arsed mechanic’.