It isn’t. It’s in reference to “having a stick up your ass”, which means being uptight and overly serious. Because they are stiff, as though a firm rod had been run the length of their body.

People also mistakenly believe that to be a homophobic phrase too. Mind you, there’s a slim distinction here. Telling someone to “shove it up your ass” could be considered homophobic, though I think of it as just rude. I feel like context would matter a lot here.

The youth today think everything insulting must be a slur of some kind and are obcesse with moral purity.