Green’s Dictionary of Slang

1. (US Und.) dismissing the victim after a confidence trick has been concluded successfully [SE round up, to conclude].

1909

W. Irwin Confessions of a Con Man 145: The third stage of the broads is known [...] as ‘the round-up.’ It consists in getting rid of the sucker in such a manner as to cause the least possible trouble.

2. a get-together, an assembly, a collection of bits, e.g. food left on a plate [round up v.].

1920		Ade Hand-made Fables 154: There was a customary Round-Up between 5 and 6 p.m. The Regulars would drop in on their way Home.
1943		R. Llewellyn None But the Lonely Heart 122: ‘What’s seventy five per cent?’ Marjoriebanks says, having a good old round up on his plate with a bit of bread.
a.1966		‘The Skewbald Black’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 120: I thought one spring, just for fun, / I’d see how cowpunching was done, / So just as the roundup had begun / I tackled the cattle king.
1967		P. Thomas Down These Mean Streets (1970) 15: I had a kid hold my head in his lap while I spoke about leaving for the last roundup in the ranch house up yonder, an idea I got from a Johnny Mack Brown cowboy flicker. It was swell acting.
1995		G. Pelecanos Down by the River 12: The Post grouped the violent deaths of D.C.’s underclass into a subhead called ‘Around the Region’; local journalists sarcastically dubbed this daily feature ‘the Roundup’.