Green’s Dictionary of Slang

(US) in carnivals, the promotion speech designed to draw the audience to one’s show; also attrib.

1929		A.T. Rogers ‘Casual Observer’ in Charlotte Obs. (NC) 24 Feb. 8/7: Show managers [...] squawk about how the adjoining fronts crab the bally.
1953		F. Brown Madball (2019) 10: [T]he punk would doss down under one of the bally platforms.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Bally or Ballyhoo — The ‘Bally’ is the ‘outside talker’s’ spiel drawing a crowd (called a ‘tip’) to see a sideshow. The bally is a sophisticated commercial, usually illustrated with quick appearances by the performers featured in the show.