GR translation: the usual loud-mouthed American ass-banditry

[Mike Jing]

V22.32-36, P23.12-16  The map does puzzle Tantivy. It cannot be put down to
the usual loud-mouthed American ass-banditry, except as a fraternity-boy
reflex in a vacuum, a reflex Slothrop can’t help, barking on into an empty
lab, into a wormholing of echoing hallways, long after the need has
vanished and the brothers gone to WW II and their chances for death.

What does "ass-banditry" mean here?