A "retired banker" refers to someone who used to work as a banker but has now stopped working and is no longer employed in that profession. Full definition
And by virtue of being in the right place at the right time after a disappointing failure, friends and upstart business partners Charlie Geller (John Magaro) and Jamie Shipley (Finn Wittrock) decide to hitch their fortunes to Burry's projections, needing the intel of a disenchanted and retired banker Ben Rickert (Brad Pitt) to get a seat at the front table. (moviemanjackson.com)