Green’s Dictionary of Slang

fix n.2

[20C+] (orig. US Und.)

1. (also fix-it, fix-up) any corrupt deal, a bribe, a favour.

2. the person who makes such deals.

In phrases

put the fix in/on (v.) (also have the fix in)

to ensure a plan or event favours whoever has paid the bribe, arranged the deal etc.