"Say what you really think"

Is anyone familiar with the ironic phrase, "say what you really think" or "Say what you really think, why don't you?" usually following a very blunt statement? It's usually something like this:

"John is a complete and utter fool and his administration of this enterprise has been an disaster of biblical proportion."

"Say what you really think, why don't you?"

It seems to be making a few inroads here in the US. I always think of it as a British phrase.

Anyway, I was wondering where it comes from. It sounds like a comic's signature line but I haven't been able to find any trace of it, at least online. Maybe it's too subtle for Google. Any ideas?