Is this an idiom: a tall order?

Hi, a tall order is an idiom, isn’t it?

Yes, it is.
Something not possible to do or comply with.
A tall story is one which you can not believe even if you want to.

Hello Mr Krishnaswamy! Thank you for your answer. So are these two idioms? I mean a tall order and a tall story? Does this mean if somebody tells me a tall story I should not believe him? Thank you again. Your help is good.