Green’s Dictionary of Slang

1. to bribe, to corrupt; of a player, to lose a game; of a horse, to interfere with its ability to win a race.

1893		M. Philips Newspaper 212: A glance suffices to tell him if a horse has been ‘stiffened’ and clever indeed must that jockey be who can pull or misride a horse without his eager eye detecting it.
1894		Truth (Sydney) 30 Sept. 4/6: For all the ‘roping’ and ‘stiffening’ of potential frauds [that] took place.
1903		J. Furphy Such is Life 230: There was nothing for it but to stiffen Toby. Toby was to get a note if he won the big event, and nothing if he lost; but it paid us to give him two notes to run cronk.
1910		Bulletin (Sydney) 6 Oct. 10/3: [T]hey had received 2s. 6d. each to run stiff, and had been entrusted with treasure to the amount of 7s. 6d. with which to stiffen three other footballers. As one of the men who was to turn dog for 2s. 6d. was a crack player, [...] it was felt that the square thing hadn’t been done.
1917		‘Banjo’ Paterson ‘The Oracle’ in Three Elephant Power 18: That horse was stiffened just after I seen you, and he never tried a yard. Did you see the way he was pulled and hauled about at the turn?
1931		D. Runyon ‘A Nice Price’ in Runyon on Broadway (1954) 193: If there is any chance of stiffening a couple of the Yales [...] we can have a little help in the race.

2. to swindle, usu. in passive.

1895

‘Banjo’ Paterson ‘How the Favourite Beat Us’ in Man from Snowy River (1902) 167: But gone is my glory, I’ll tell you the story / How I stiffened my horse and got stiffened myself.