Green’s Dictionary of Slang

1. (Aus.) on good terms, supportive.

1978		S. Murray Legionnaire 220: Fonfon comes to the tent and brings steaks and lamb chops from time to time and I think this tips the balance and just keeps Subera on side .
1999		P. Temple Black Tide (2012) [ebook] The question you want to ask, Tony, is this: am I better off square with the Armits and onside with Bren, or one-sixty deep and offside with Bren?
2006		B. Matthews Intractable [ebook] Pretty soon word was out and an unofficial stamp of approval had been delivered. The crims were onside.
2019		C. Hammer Silver [ebook] ‘We need her onside’.

2. (UK Und.) suborned, in one’s pay.

1986		R.G. Barrett Real Thing 85: I’ve got the two top coppers on side up there.
1998		P. Doyle (con. late 1950s) Amaze Your Friends (2019) 183: ‘[Y]ou’re still working for me [...] I want you onside. You’re the only one who got a result’.
2007		N. ‘Razor’ Smith Raiders 270: Dex had some of the staff of the hotel onside – he knew the value of flashing a bit of cash.