Oh that’s easy, “underfull /hbox” is like “overfull /hbox”, just the other way around

What’s “overfull /hbox”, you ask? Oh that’s easy, it’s like “underfull /hbox”, just the other way around

Yeah LaTeX definitely has some of the jankiest log outputs of any program. I just looked it up actually and it’s basically a warning that that but it the document might not look great (I guess something like in newspapers with justified columns where you sometimes see s t r e t c h e d w o r d s.) Definitely insane to print warnings like that by default. Maybe it made more sense in the 70s?