From Discworld & Terry Pratchett Wiki
A tosheroon is the name for a lump of debris and rubbish made of the mud and gunk found clogged in drains. While they appear worthless, tosheroons can contain valuable items, such as old coins, lost rings and alike. One man who knew the value of such things was Harry King, who as a boy scavenged for valuable rubbish across the city. Harry remembers the day he found his first tosheroon, and that it was taken from him by a bigger man. But he didn't cry and make a fuss, as a knew what they looked like, and he knew that he could find more.
Eventually Harry had found many tosheroons and sold them for a profit, building up a mini-empire of scavenging street kids. As a reward to himself on his birthday, he hired a couple of trolls to visit the man who had stolen his first tosheroon and beat the &%£$ out him.
Harry tells this story to William de Worde, impressed that he knows what the word means. When William claims that his 'tosheroon' is going to be taken from him by the bigger boys, Harry is convinced to help, and agrees to sell them the printing paper they require, on the condition that they provide press coverage for his daughter's wedding. Having later read (or had read to him) a copy of the news paper, Harry can see that William is like him in some ways - willing to sift though muck in order to find the valuable truth.
Tosheroons also figure large in the non-Discworld novel Dodger.