Green’s Dictionary of Slang

1. lit. or fig. horsepower.

1902		‘Banjo’ Paterson ‘Song of the Artesian Water ’ in Rio Grande’s Last Race (1904) 70: Now, our engine’s built in Glasgow by a very canny Scot, / And he marked it twenty horse-power, but he don’t know what is what: / When Canadian Bill is firing with the sun-dried gidgee logs, / She can equal thirty horses and a score or so of dogs.
1956		‘James Updyke’ [W.R. Burnett] It’s Always Four O’Clock 158: He was trying to ape us—the Royal Trio—and he just didn’t have the horses.
1979		‘Iceberg Slim’ Airtight Willie and Me 33: He stomped the horses and blasted off.
2001		G. Pelecanos Right As Rain 49: It was your basic Taurus, outfitted with more horses than as legal, more juice than Ford used to put in its high-horse street model.

2. con. with the, horse-racing.

1933		S. Walker Night Club Era 33: ‘Just tell the boys not to place any more bets on the horses over the phone’ .
1968		W.S. Hoffman Loser 146: ‘[T]hree years ago I embezzled eighty thousand dollars from the company I work for to play the horses’.
2013		T. Peacock More You Bet 15: ‘The horses’ ( which were and are also known as ‘the gallops’, or ‘the nags’, or ‘the neddies’ or the ‘gee-gees’.