Green’s Dictionary of Slang

1. sophisticated, upper-class.

1890

Sporting Times 1 Mar. 2/3: A son of the Duke of Belle-view. / Thoroughbred down to his heels, sir, / In spite of his dare-devil ways.

2. admirable, dependable, trustworthy.

1905		‘Doss Chiderdoss’ ‘An Improved Understanding’ Sporting Times 14 Jan. 1/4: We each trust one another, we’re both thoroughbred; / I’m hard up for a fiver, and loan sharks I dread.
1979		‘Iceberg Slim’ Airtight Willie and Me 30: I was about to tell him what a thoroughbred, stand-up Nigger he was.

3. (US Und.) used by pimps to describe a first-rate whore.

1968		N. Heard Howard Street 137: I’m talking about a stomp-down sophisticated thoroughbred whore like my woman.
1973		D. Goines Street Players 10: Let’s ride down and see what them thoroughbred bitches I got are doing.