is that so informal 1. Oh, indeed? That's interesting.
■ Used in simple acceptance or reply.
"The Republicans have pulled a trick at city hall." "Is that so?" 2. Surely not?
■ Used in disbelief or sarcasm.
"The moon is made of green cheese." "Is that so?" "I'm going to take your girlfriend to the dance," said Bob. "Oh, is that so!" said Dick. "Try it and you'll be sorry." [TahlilGaran] English Idioms Dictionary ▲