is that a fact?
Also, is that so? Phrases indicating that one is following what another person is saying. These expressions, which require no reply, can be used either straightforwardly, as in You mean you've flown to Paris three times just this month? Is that a fact? or sarcastically, expressing disbelief or contempt, as in Just wait, I'll be promoted over you before the year is out.
Oh yeah, is that so? [Late 1800s]