racehorse, n. — Green’s Dictionary of Slang

racehorse n.

1. (Aus./N.Z.) in sense of something sleek and lean.

(a) a very thin roll-up cigarette.

1953		K. Tennant Joyful Condemned 165: The tough lare rolled another ‘racehorse’ cigarette and tossed it to David.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxviii 10/3: racehorse: A very thin cigarette. More the rule than the exception in nick.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Racehorse. A rolled cigarette containing only a few scraps of tobacco. A pregnant racehorse is a cigarette swollen in the middle.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 151/1: racehorse n. a thin-rolled marijuana joint.
2003		McGill Reed Dict. of N.Z. Sl.
2013		D. Whish-Wilson Zero at the Bone [ebook] Andrews [...] building a White Ox rollie, a regulation prison racehorse, thin and tight out of habit.
2020		D. Whish-Wilson Shore Leave 46: Ryan finished with his makings [...] and gave fire to the racehorse-thin rollie.

(b) a thin pack.

1965

R.H. Conquest Horses in Kitchen 46: A hobo’s swag was known as a ‘racehorse’ swag—long and lean.

2. (US) an up-market prostitute, but note cit. 1969, which implies turnover rather than quality [on pattern of thoroughbred n.].

1969		B. Jackson Thief’s Primer 188: She’d made $800 where a young what-they-call ‘racehorse,’ she’d have run in there, got her $20, and have been back in fifteen minutes.
1971–2		C. Shafer ‘Catheads [...] and Cho-Cho Sticks’ in Abernethy Bounty of Texas (1990) 212: race horse, n. – a woman who makes a lot of money; usually a prostitute.
1990		S. Morgan Homeboy 50: Once she was a racehorse, a pricey L.A. pro.