Green’s Dictionary of Slang

a bathing costume.

1929		Daily Tel. 3 June 7/1: Two coloured swimmer with brassiere attached [OED].
1967		Sun. Truth (Brisbane) 23 July 1/1: Bikini girls at Parliament House [...] when a parade of new season’s swimmers [...] will be on show .
1978		Courier-Mail (Brisbane) 22 Feb. 1/9: I am not an exhibitionist and if I go swimming on the main beach, I would wear swimmers .
1996		A. Close Official and Doubtful 33: The bimbo on the desk will lend you your swimmies if you tell her you’re with me.
2004		L. Redhead Peepshow [ebook] I suppose you could still be useful for the odd bit of surveillance, long as you don’t wear those sparkly swimmers .
2018		C. Hammer Scrublands [ebook] The Riverina sun has leached the red from her swimmers.