Wikipedia has an article about the “Lexicon of Comicana” here, which includes a number of these terms. Many of them actually appeared a few years earlier in his Backstage at the Strips. My impression is that most of these were fanciful inventions on his part, but have been adopted by many cartoonists due to Walker’s reputation and influence on the field, and the usefulness of having names for these things. (It’s easier to say “plewds” than “drops of sweat flying from the character’s face to show that he’s nervous or working hard”.)
The Wikipedia page also shows an illustration of squeans.
