stick in the mud

noun2

1. •
   A person unwilling to participate in activities; a curmudgeon or party pooper.

   • Have a little fun sometimes and don't be such a stick-in-the-mud.
2. •
   More generally, one who is slow, old-fashioned, or unprogressive; an old fogey.

   • There’s a parallel here: the idea of Barbie dolls being in any way noxious seems, in the current climate, to be as hopelessly stick-in-the-mud a stance as the concept of selling out. — 2023