Green’s Dictionary of Slang

1. to add as a bonus, to throw in.

1806		G. Colman Yngr John Bull IV i: He paid the old man’s debts, and the bailey’s broken head ware chuck’d into the bargain.
1873		J. Greenwood In Strange Company 208: When they’re reg’ler [...] we chucks in Sundays.
1884		‘’Arry at a Political Pic-Nic’ in Punch 11 Oct. 180/1: Rosherville Gardens chucked in, with a dash of the Bean Feast will do.
1892		H. Nisbet Bushranger’s Sweetheart 35: I know Melbourne outside and inside, with its suburbs chucked in.
1954		Wodehouse Jeeves and the Feudal Spirit 30: From the fact that he had not chucked in a lot of ‘Sirs’ I presumed he had been talking to some pal of his.
1965		F.J. Hardy Yarns of Billy Borker 26: Champagne and a chicken dinner chucked in.
1999		Guardian G2 10 Aug. 16: Chuck in Biggles and James Bond [...] to make sure we get our money’s worth.

2. to contribute (cf. chip in v. (1)).

1924		G.H. Lawson Dict. of Aus. Words And Terms 🌐 CHUCK IN – To contribute.
1936		D. Stivens Tramp and Other Stories 6: Still, he’ll chuck in a few pounds!
1965		J. O’Grady Aussie Eng. (1966) 27: There’s a keg on tonight—we’ll all be chuckin’ in.