I'm not sure it actually in any of the books. It's just (I think) a generally Discworldy idea that was approved by Terry (as were all the Clarecraft items). I guess it's a somewhat tongue in cheek way of saying "poison" in the same way that Dibbler's dragon dectors were basically a piece of wood that, if ignited meant you'd found a dragon.
Thinking about it - sometimes these Clarecraft figures came with a little card with the product number and, if I remember correctly, a quote from one of the books to the relevant passage from the book where they appear.
I have this particular item and a Dibbler's compass, that I'm not sure is mentioned in the books either.