A 'bedwetter' is someone who has bladder control problems and accidentally urinates during sleep.
The phrase is often used as an offence; referring to someone of adolescent age or older who is also deemed to be a pathetic individual with little assertiveness, moral fibre or 'backbone'.
Such individuals are often commonly called:
Fags, mommy's boys (US English), mummy's boys (UK English), bitches or bitch-boys.
Bob: "Clive drives a shit-ass French car!"
Dan: "So why not say it to his face?"
Dan: "'Cause you're a bedwetter. Bitch!"