Green’s Dictionary of Slang

(US) second-rate.

1754		Trial of Elizabeth Canning in Howell State Trials (1816) 563: I had a scrub-lawyer with me, and, I believe, there are a great many of them in town.
1842		Whip & Satirist of NY & Brooklyn (NY) 5 Mar. n.p.: Wilkes, the late scrub editor of the Flash’.
1907		W.M. Raine Wyoming (1908) 65: It’s a scrub bunch. We want to get rid of them.
1911		E.B. Morris Sophomore 7: Bannister. They played like the scrub team from an old ladies’ home.
1944		T. Thursday ‘Base on Balds’ in Sports Winners Spring 🌐 We’re scheduled to play a scrub team with the high-sounding name of King Crackers.
1950		A.B. Guthrie Way West 50: Good God! Scrub stock. Marry into scrub stock, and it was all right! Call Hank McBee pappy and Mrs. McBee ma.
1977		Fidrych & Clark No Big Deal 33: ‘I really wasn’t into baseball. [...] I played it—yeah, Little League, pickup scrub teams, y’know’.
2008		G. Pelecanos Way Home (2009) 55: The midnight shift, who the boys considered to be the scrub members of the security team.