a brain that needed a good brisk washing
if Boston AA is a cult that like brainwashes you, he guesses he’d got himself to the point where his brain needed a good brisk washing, which Lenz knows is not an original view, being exactly what big blockheaded Don Gately repeats about once a diem
Context
Green's flat answer to the cult accusation is the Program's whole rejoinder in one shrug: if AA brainwashes, then his brain had gotten itself to the point of needing exactly that. The book marks it as unoriginal on purpose — it's what Gately says about once a day — because the wisdom that saves is meant to be a cliché, the un-clever surrender that the too-clever (Lenz) can only sneer at.