Dear users!

I am Czech, therefore I read Pratchett´s books in Czech. Now I read The Colour of Magic in English. I do not understand one thing. Please, help! Twoflower tries to explain Rincewind his job. He talks about inn-sewer-ants polly-sea. That is clear - insurance policy. But later on he talks about "reflected-sounds-of-underground-spirits". I guess, this should work the same way as inn-sewer-ants - just words replaced with another ones, that sound almost the same. But I can neither figure out what those original words should be, nor I can find an explanation on the Internet. Does anybody of you know?