Green’s Dictionary of Slang

a pick-up, a casual sexual partner; thus casual sexual intercourse.

1831		Satirist (London) 4 Dec. 277/2: Lord Hertford of gambling is weary, / And now only plays the love-game; / Is fixed to, though fond of a deary, / A Stray-un—at least so by name.
1884		Randiana 73: Clinton, you are [...] such an incredible cunt hunter. [...] There may be some stray ‘stuff’ drop in while we are there, but I warn you not to try it on with Mrs. Leveson.
1937		J. Worby Other Half 238: ‘Well, you know how it is, Bonzo,’ he replied. ‘A bit of a stray.’ ‘Yeah!’ I said. ‘That bit of a stray is my dame.’.
1962		F. Norman Guntz 29: I started [...] picking up bits of stray all the time.
1976		D. Ireland Glass Canoe (1982) 205: I’d latched onto a bit of stray stuff.
1985		B. Humphries Traveller’s Tool 85: Amsterdam is chock-a-block with Art Galleries too and there is no better place for picking up a nice bit of stray.