It changes for every character. I actually had many as TBN for a while on my notes, but sometimes it comes from a word I hear/mishear, a movie, or just something fun I find in Etymonline.
Mortimer and Dylan mean "still water" and "tide" respctively. Mort's name is the oldest thing in the entire comic creation process so I no longer remember why, but I chose Dylan's name to contrast in both personality and approach to things, while still being the opposite side of the same coin as they got along well despite that. (as a bonus, their father is named Windebrook, a brook being the middle term between them)
Others have... Far weirder processes. All the Amberhobs' names come from the single scrapped character that they came from named Claws, evolving into a naming system for the entire species of a gem+sharp object or just the latter. Veriesin is a combination of Vera and Taliesin because I was reading a lot about celtic mythology that week. High Priestess Dismacyndra is by far the weirdest, being the combination of my favorite Final Fantasy XII enemy and a mispelling of Cynder from the Spyro series I heard one time.
My main username, Iris-Grimoire, came from a long series of attempts to name something like the term "purple prose". Eventually adopted it as an artifact my mascot, the little dragon wizard in my icon, uses.
extra: a character from the same universe as Splitting Image but not in the comic, Master Sufreed, is named after what I hear in Rivers in the Desert from Persona 5 in place of "so perseverent". I still can't tell that's actually being said.