What IS the Secret of Monkey Island?
Although never officially confirmed by Ron Gilbert, we have it from reliable sources that this article is pretty dang close to explaining the whole thing.

Why the heck is Guybrush afraid of porcelain?
Our interview with Chris Purvis may (or may not) clear that up.

How long has TSB been around?
Since 1996.

How did Guybrush get his name?
The 2020 update.

In the late nineties, we claimed in this very FAQ that Guybrush was named after the generically labeled character-graphics Deluxe Paint file guy.brush. Guy because the main character was a guy, and the extension indicating it was a brush file.

In a scathing indictment of our reporting, Ron Gilbert posted a long retort about the real name of the file, and thus the origins of Guybrush’s name. A quote:

During the early days of Monkey Island I didn’t have a name for Guybrush. We just called him the "guy".

When Steve Purcell was doing concepts for "the guy" he was doing them in dpaint. In dpaint you could select a section of the screen called a "brush" and save it out.

It was these files I got from Steve. I saw the file names so many times that the name "guybrush" stuck.

We maintain the crux of our FAQ was correct, but apologize for the mistakes made with regards to the file extensions. And make sure you read the full secret behind Guybrush’s name.

(Incidentally, Tim Schafer wanted to name the character Hank Plank. Source: Tim’s tweets.)

Back to the nineties FAQ:

Threepwood was decided in a company contest – the name comes from the book "The Brinkmanship of Galahad Threepwood" (US title). "Threepwood" is also rumoured to be the name of Dave Grossman’s character in RPGs.