Green’s Dictionary of Slang

one who swears false oaths; a perjurer; thus mounting, perjury.

1781		G. Parker View of Society II 23: There is another set of Queer Bail [...] who are distinguished by the name of Mounters. They are so denominated from the party’s borrowing the clothes which he wears when he goes to give Bail. There are houses which lett out wigs and coats for this purpose; the wigs are well-powdered, and the coats large to buttun up to the chin. Mounted in this tradesman-like manner, these Bail are brought to Court.
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.: mounter a common perjurer, false evidence, and one who becomes bail for hire [...] Mounting false swearing.
1809		G. Andrewes Dict. Sl. and Cant n.p.: Mounter a common perjurer; villains who give false evidence, and become bail for fellows of their own society [...] Mounting false evidence.
1812		Vaux Vocab. of the Flash Lang.
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.
1850		Fast Man 7:1 n.p.: Michael Edwins, a noted bill negotiator, swindler, and mounter (false swearer).
1859		Matsell Vocabulum.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1895		C. Crowe Aus. Sl. Dict. 50: Mounter, a false swearer from Mount, to swear falsely.