I can’t seem to find the history of it, but it essentially means somethings a bit rubbish, bad, or crap.

Pants are typically underwear in Britain, we wear trousers (Jeans, chinos, suit trousers) over our pants (boxers, y-fronts, briefs) so probably not something you’d want to show off that much.

Thanks for the reference to pants. In US pants covers jeans, chinos, slacks. Shorts are well, shorts. Underwear is our boxers, briefs, tighty-whiteys, etc.

A lot of Brits will use pants the same as you do to be fair, myself included.

Shorts are mostly just shorts, but we do have jorts and chorts depending on how hipster your shorts wearing cohorts are.

I believe it's this one?

- "adjective. British slang. Not good; total crap; nonsense; rubbish; bad "The first half of the movie was pants but I stayed until the end and it was actually a great film.""

https://www.urbandictionary.com/define.php?term=pants

HN is the pants, man! (am I doing it right? am I cool?)

Better would be - HN is the dogs bollocks, other sites are just bollocks

No, it's the cat's pyjamas.

refers to underpants, with some perhaps dubious stains. common uk english.

Thanks for that. I figured is was UK but didn't want to assume.