Green’s Dictionary of Slang

1. to perjure oneself for money.

1789		G. Parker Life’s Painter (1800) 145: These kind of men attend the courts of law; their price is five shillings for what they call mounting; they have been known to mount two or three times in one day.
a.1790		H.T. Potter New Dict. Cant (1795) n.p.: mount to give false evidence.
1801		G. Hangar Life, Adventures and Opinions II 60: Those necessary professional accomplishments, such as [...] how to scamp, prigg, floor, [...] mount, lumber, and fence.
1809		G. Andrewes Dict. Sl. and Cant.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 253: mount: to swear, or give evidence falsely for the sake of a gratuity. To mount for a person is synonymous with bonnetting for him.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1859		Matsell Vocabulum.

2. (US black) to brag, to boast, to attack verbally; thus mounting n.

1964		R. Abrahams Deep Down In The Jungle 48: A verbal attack is called ‘mounting,’ or ‘getting above,’ that is placing the other in the female position.
1972		T.M. Kochman ‘The Kinetic Element in Black Idiom’ in Kochman Rappin’ and Stylin’ Out 169: Contest, as in ‘whupping your game,’ ‘mounting,’ ‘scoring’.