shower British informal
a group of people perceived as incompetent or worthless
I think this term is becoming obsolete. It's certainly not something I've heard in the street recently. The first place I encountered it was the 1959 film I'm Alright Jack.
Major Hitchcock: They're a shower. Absolute shower.
According to wiktionary it comes from shower of shit but there is no justification given.
Best Answer
No less an authority than Eric Partridge seems to think it probably came from shower of shit. Which is what I usually say on the few occasions I use the expression in any form, though I'd always assumed of shit was a jocular alliteration with some pre-existing meaning of shower.
I don't know for sure if Partridge et al are right, but if they are it puts shower on a par with berk as being more coarse than many speakers realise.
Until I just looked it up there, I'd never come across what he suggests as the "full" original form - shower of shit from Shropshire.
Like berk, shower might not be the latest cool slang, but I think it's in no danger of becoming obsolete. Especially for those who need/want to insult Shropshire folk!