Green’s Dictionary of Slang

‘to play improper tricks with [a horse] in order to make him lively’ (Hotten, 1860); for detail see cit. 1823.

1793		Sporting Mag. July II 221/1: The whip before, and the aggravating stimulus of the ginger behind (better understood by the appellation of ‘figging’).
1804		Sporting Mag. Mar. XXIII 351/2: His horse’s cock’d tail, / Shows that all wont avail; / For, by Jove, there is no need of figging.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 76: Fig, figged—ginger; little lumps whereof are thrust into the rectum of horses to give them a short-lived vigour; they are then said to be figged, and carry better while the stimulus lasts; but horses of any original breeding afterwards flag in their disposition, as if resentful of the beastly indignity shewn them. Fellows there are who traverse Smithfield of Friday evenings seeking for old figs.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1876		C. Hindley Life and Adventures of a Cheap Jack 258: It was ‘jigged, digged, and figged,’ and as the horse was being run up and down by a jockey cove, Tom kept saying to the farmer, ‘You wont buy him, he’s got a nasty nose’.
1887		W.E. Henley ‘Villon’s Straight Tip’ in Farmer Musa Pedestris (1896) 176: Suppose you screeve, or go cheap-jack? / Or fake the broads? or fig a nag?