Noun.
A portmanteau for the slash or yaoi pairing between Sherlock Holmes and Dr. John Watson, from the original Sherlock Holmes books written by Sir. Arthur Conan Doyle.
Sherlock + Watson = Shwatsonlock

It can be argued whether Shwatsonlock is canon or merely fandom, but for this particular definition, I will simply describe it as fandom.

Usually, the term is used to describe a genre/pairing of Fanfiction stories, or Doujinshi.

There are a variety of different series/versions of movies and television shows made of Sherlock Holmes. So, while Shwatsonlock may define the general fandom romantic/sexual relationship between Sherlock Holmes and John Watson, the interpretation of these characters in fan works will vary from one series/version to another.

For example,
The Robert Downey Jr. interpretation of Sherlock Holmes is VERY different from that of the Jeremy Brett interpretation of Sherlock Holmes.

As such, when reading a Shwatsonlock story/comic, make sure to become aware of the version that the particular story/comic is based off of. Sometimes it makes a difference, sometimes it doesn't. But that is simply a small mindful note that I personally deem important.

When you eat dirt for the first time and leave your family to build a shack in the woods