Green’s Dictionary of Slang

to use any form of trickery, esp. the burning of marks into the teeth, in order to reduce the appearance of a horse’s age; thus bishoping n., the use of such trickery.

1727		R. Bradley Family Dict. I n.p.: Horse This way of making a horse look young, is by the Horse Coursers called bishoping [F&H].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1801		‘Modern Dict.’ in Sporting Mag. May XVIII 98/2: Bishoped, or To Bishop – A term among horse-dealers, for burning the mark in a horse’s tooth, after he has lost it by age.
1811		Lex. Balatronicum.
1821		‘An Amateur’ Real Life in London I 246: If you have a horse you wish to dispose of, the same school will afford you instruction how to make the most of him, that is to say, to conceal his vices and defects [...] to alter his whole appearance by [...] Bishoping, blistering.
1839		Hereford Times 15 June 4/3: A term among horse-dealers for burning the mark into [a] horse’s tooth [...] by bishoping a horse is made to appear younger than he is.
1884		Illus. London News 23 Aug. 171, col. 2: To bishop... a term... signifying the use of deceptive arts to make an old horse appear like a young one [F&H].