1 Ponsonby: Do you mean to tell me that we have no cannon on board?
2 Mate: We’ve never had any cannon on this ship, sir.
3 Ponsonby: How can that be?!
4 Mate: The pirates are the protagonists of this story. Everything canonical happens over there!

The word "canon" has several meanings. The one used as the punchline here is a sense that has become much more common in English usage in recent years, with the explosion of literary/cinematic/fictional settings with multiple stories set within them. In this sense, the canon is the accepted set of characters, events, and stories that occur within a particular fictional universe. Exactly who is doing the accepting can vary, from being either fully under the control of a single author/creator, to being a shared fictional creation with multiple authors contributing canonical works, to a canon that is defined purely by the consumers and fans of a particular setting, who may wish to include or reject particular details within their own personal definition of what is canonical or not.

The word "canon" was originally applied in a similar sense to Biblical canon, the set of Biblical texts regarded by various religions or religious scholars as being the authoritative Word of God. All of the various Jewish, Catholic, Protestant, and Eastern Christian churches and sects who use this material as holy scripture regard different partially overlapping subsets of the material as canonical, while rejecting the remainder.^[1]

The transfer of the term "canon" to referring to an authoritative set of fictional stories^[2] was first applied to the Sherlock Holmes stories, of which only the ones written by Sir Arthur Conan Doyle are considered to be canonical, thus excluding all later stories written by other authors. This usage seems to have been started with Ronald Knox, an English Catholic priest and aficionado, critic, and writer of detective fiction, thus perfectly qualifying him for the crossover of a term from the sacred domain to the profane.

In another interesting crossover of religious and literary careers, Knox is also known for writing the "Ten Commandments of Detective Fiction", which lay down rules for writing a whodunnit story in a way that presents a fair, solvable, and ultimately satisfying puzzle for the reader.^[3]

[1] What this implies about the ultimate veracity of the beliefs of any particular church is left as an exercise for the reader.

[2] Or other authoritative sets of fictional stories, depending on your point of view.

[3] If only the writers of the multiple conflicting Biblical canons had followed these rules, there might not be so much disagreement over what it all means.