Green’s Dictionary of Slang

to the best extent possible.

1839		Albany Microscope (NY) 9 Mar. n.p.: He flatters himself that things will work about right.
1849		Manchester Telescope (NH) 10 Feb. n.p.: It does up the thing about right on the loafers who stand on the corners to watch the girls.
1853		Life in Boston & N.Y. (Boston, MA) 8 Oct. n.p.: Miss Allen [...] showed her legs ‘about right’.
1864, 1867, 1870		Hotten Sl. Dict.
1909		J. Ware Passing Eng. of the Victorian Era 209/1: Right, About (Peoples’). Modest self-depreciation, or depreciation in general; not absolutely right, but nearly so, e.g., ‘I thrashed him about right’.