It’s in Green’s and HDAS, a slang term meaning something excellent or first rate.
From Green’s Dictionary of Slang:
1908 T.A. Dorgan in Zwilling TAD Lex. (1993) 96: What’s good today Jim. Our beef pie today is catsy —It’s a darb order. 1913–14 Van Loan ‘Scrap Iron’ in Taking the Count 213: Your right cross is a darb.
There’s a slightly older sense of the word from US underworld slang meaning money:
1904 Number 1500 Life In Sing Sing 261: As I had plenty of the darb I blew away and beat it back to Chic, and framed in with a couple of guns who were working east on the rattlers. 1919 ‘Bugs’ Baer in Atlanta Constitution 23 Nov. A2/4: Back to the office, where Citronella breaks out a pair of dice. […] Simpie cops all the Darb.
Darby, also meaning money, is much older, dating to the seventeenth century. That’s almost certainly the source for the money sense of the clipped darb. Green’s suggest that dab, meaning a skilled person or expert, which also dates to the seventeenth century, may be the source or had an influence on the sense meaning first-rate thing.