The landlord, who has been persuaded to be friendly and hospitable, and who has previous experience of troll beverages, produces a tankard full of eye-wateringly strong acid, possibly vinegar. One half of the shilling is dropped into the vinegar where it immediately starts to dissolve. The other half is traded in for small copper coin. In the manner of a conjuror performing a trick, the unibrowed landlord holds up a copper coin for inspection, then drops that into the foul brew, which raises a suitably foaming head.
Carborundum braces himself, then drains it in one. People are swiftly advised not to stand immediately behind the troll, who falls over backwards and is dead to the world until the next morning.
It is clear that the Floorbanger has a lot in common with luglarr.