Green’s Dictionary of Slang

(US Und.) highly competent.

1931		Irwin Amer. Tramp and Und. Sl. 60: Darb. – Unusually skilled or able; excellent.
1935		A.J. Pollock Und. Speaks 29/2: Darb, satisfactory.
1962		Ragen & Finston World’s Toughest Prison 796: darb – Unusually skilled or able; excellent.