Green’s Dictionary of Slang

(Polari) a well-executed confidence trick.

1821		Egan Life in London (1869) 101: A hand truck was procured, and drugged Charley (watchman) and his box were then transferred to another locality, so that when Charley awoke he found himself and box ready for doing duty in another parish. This trick was estimated to be a multa bona fakement.
1846		Swell’s Night Guide 57: Multa bona fakement, split me! Where’s the plant, cully?