When to use "Is that so?". One of my friend just responded with the idiom "IS THAT SO?" To be honest,whenever i hear or read this idiom as a response to me, I initially feel a bit annoyed. As far as i know, this is some kind of rude phrase but i just ignore the feeling and tell myself, "maybe the person I talk to doesn't know the meaning and usage of it " or "maybe he/she means to say it") .. whichever it is, I stick to the former to ease my annoyance. Being concern, I think it's time to correct this mistake. Not all of the people are understanding. This simple mistake may cause trouble. I must do an action before it's too late. (hihi..just exaggerating ^.^v) I made a google research to find out the real meaning of the idiom, "Is that so?". Based on http://www.wordnik.com/, it means, "Really? Is it true?". Additionally, http://idioms.thefreedictionary.com/ says it means, "That is what you say, but I do not believe you. (No rising question intonation. Slightly rude.) " See? Even the reference says it's RUDE. Isn't it awful knowing that someone doesn't believe what you say when you're actually saying the truth? Well, well, well, this is just MY OPINION. Correct me if I'm wrong. Have a good day to one and all!