Green’s Dictionary of Slang

to talk or boast .

1812		Vaux Vocab. of the Flash Lang.
1821		D. Haggart Autobiog. 26: They said, they had done nothing to mang about.
1848		Ladies’ Repository (N.Y.) Oct. VIII:37 316/2: Mang, to talk. Mangsman, a lawyer.
1859, 1860		Hotten Dict. of Modern Sl. etc.
[1869		Australasian (Melbourne) 17 July 8/3: At every step we have evidence of Hindoo origin. For instance, chur, a thief; maung, to beg; pane, water; tschib, the tongue].
1873		Sl. Dict.