Green’s Dictionary of Slang

plummy adv.

[SE plum adv., completely]

excellently; satisfactorily.

1828		‘The Song of the Young Prig’ in C. Hindley James Catnach (1878) 171: Draw the fogles plummy.
1830		W.T. Moncrieff Heart of London II i: We chaunt so rummy, / And slang so plummy.