This is an excerpt from Terry Pratchett’s book, “The Light Fantastic”
From here: http://www.electric-escape.net/pratchett/The-Light-Fantastic
What is a soap herring and why does it have a small life expectancy?
A herring is a small fish (commonly eaten either as-is or smoked (when it is called a kipper or a bloater), in Britain). If it were made of soap it would soon dissolve in the sea.
I just did a search on “soap herring” and found a lot of webpages that include it. There are some in which the word “soap” just happens to be before the word “herring.” There are a bunch of people who are just quoting the passage from the Pratchett novel. There are many cases where someone uses the term “soap herring” as if it were a standard phrase. It may have pre-dated Pratchett, or they may be using the term as a conscious reference to Pratchett. There are also a lot of webpages that use the phrase “good red soap herring, says Joe.” These pages are apparently all filled with nonsense sentences.
Yes, it’s because it will dissolve quickly. I’ve seen Pratchett use “chocolate teakettle” somewhere in a similar sense.