British slang, originating in Plymouth, Devon, it means an ineffectual male. The epithet also implies that the recipient is also moronic impotent, effeminate and
incompetent. The origin is believed to derive from the expression 'not worth a Molly's piss', where Molly or Mollie was a slang name for a
prostitute. So a Molly Piss is someone who is not worth a prostitute’s piss, meaning that they are of no account or totally worthless.