Apologies for the wetbrain hallucination, HN!
5. Print. a. a thick strip of type metal less than type-high. b. such a strip containing a type-high number or other character for temporary use. c. a line of type in one piece, as produced by a Linotype. 8. Journalism. a. a short phrase or title used to indicate the story content of a piece of copy. b. the line of type carrying this information.
Got to the Wikipedia page https://en.wikipedia.org/wiki/Slug_(publishing) which could possibly support the slimy conclusion of "it's a trail through the process" but that article has an etymology section that refers to the metal slug
I guess it could mean both depending on whether you're looking for the meaning of the word or the meaning of the concept but I didn't find any other slimy grub references (via an admittedly limited double check)
>The origin of the term slug derives from the days of hot-metal printing, when printers set type by hand in a small form called a stick. Later huge Linotype machines turned molten lead into casts of letters, lines, sentences and paragraphs. A line of lead in both eras was known as a slug.