The UK compilation DVD set has the actors superstitious chant as "Hot potato, off his drawers, pluck to make amends. Ow!". Drawers is old english for pants or trousers. This version also looks more likely if you observe the lip sync in the scene.
The character names of Mossop and Keanrick refer to three famous thespians of the 18th and 19th century: the Irish actor Henry Mossop (1729-1773), the English actor Edmund Kean (1789-1833), and the English actor David Garrick (1717-1779).
There is some debate among fans as to the exact words to the chant the actors use to "exorcise evil spirits" when Blackadder mentions "Macbeth." The subtitles on the US DVD release indicate it is "Hot potato, orchestra stalls, Puck will make amends." The original script, published in the 1998 book "Black-Adder: The Whole Damn Dynasty" states that the actors perform "a violent and stupid superstitious routine," but doesn't include their dialogue.