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In The Colour of Magic, on page 46 of my copy, Twoflower says this word:

“Reflected-sound-of-underground-spirits?”

What term is being referred to above?

It seems to be like “in-sewer-ants-policy”, i.e. a term that they don’t have a word for in the language (“insurance policy”), but I can’t figure out what it’d be.

I asked two people who couldn’t figure it out, either.

The word is "echo-gnomics" ("economics")

An echo is a reflected sound; gnomes live underground.

Etymologically, "gnomes" [mythology] are described as "a legendary race of human-like beings, usually imagined as short and possibly bearded males, who inhabit the inner parts of the earth and act as guardians of mines, mineral treasure, etc." (Source: wiktionary)

Apparently I've played too much D&D where gnomes aren't specifically tied to the underground.

Rincewind tries his hand at a better translation later in the book (emphasis added):

Bloody hell, he thought. He’s alive! Me too. Who’d have thought it? Perhaps there is something in this reflected-sound-of-underground-spirits? It was a cumbersome phrase. Rincewind tried to get his tongue around the thick syllables that were the word in Twoflower’s own language.

“Ecolirix?” he tried. “Ecro-gnothics? Echo-gnomics?”

That would do. That sounded about right.

As with 'insurance', the concept of 'economics' (a reflected sound is an echo, underground spirits are gnomes) is largely unheard of in that part of the disc. Rincewind describes it as 'financial wizardry' (emphasis added):

“Well, my point is, you see, that gold also has its sort of magical field. Sort of financial wizardry. Echo-gnomics.” Rincewind giggled.