<isabella>
After more exploration I found out that Fortran 77 (available in the same bsd repo) also called what is now known as file descriptors "units"
<isabella>
And David Korn was a maths guy who wrote and published a bunch of Fortran code
<isabella>
>Input/output is done directly by referencing a file or device by its name (FDname) or indirectly by referencing a logical I/O unit (SCARDS or INPUT, SPRINT or PRINT, SPUNCH or OBJECT, GUSER, SERCOM, 0 to 99). FDnames are assigned to logical I/O units using keywords in the command language or by default.
<isabella>
So tldr: -u was picked by someone who started programming before UNIX existed and when things that we now call file descriptors were widely known as units
<isabella>
Relics from like 60 years ago
<jetchisel>
Ancient relics of ze past!
<jetchisel>
Or it was created/picked by the same folks who wrote Unix.