Green’s Dictionary of Slang

a generous person, one who pays up without complaint, thus antithesis, non-parter, bad parter.

1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1890		‘’Arry on Equality’ in Punch 22 Feb. 85/2: And there’s lots o’ the rich is good parters, bit here and bit there, dontcher know.
1895		Crowe Aus. Sl. Dict. 56: Parter, a liberal person; bad parter, a bad pay.
1939		Wodehouse Uncle Fred in the Springtime 82: [H]e had come up against one of the Untouchables, a man to whom even Oofy Prosser, that outstanding non-parter, would have felt compelled to raise his hat.