Where does the pejorative meaning of "shower" come from?

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!

A little surprisingly, ‘Chambers Slang Dictionary’ gives shower only as Australian slang for a dust-storm. It does, however, have a separate entry for shower of shit with these three definitions:

1. an unimpressive group of people; occas. an individual. 2. A pile of second-rate things. 3. A term of abuse aimed at a single person.

As alternatives, the entry also provides these delightful terms: shower of cunts, . . . savages, . . . shites, . . . tom tits, . . . whore’s bastards .

The US has the expression 'shit shower', meaning when all went wrong, or we can label a person or persons as a 'shit shower', meaning they can do abs. nothing right.