literally speaking it is just another way of saying a very commonly and aptly used phrase "NO SHIT". it is when stating the blatant obvious and saying it as if it was just thought of on the spot. It is used to show distaste in a rather sarcastic and underhanded way and if said properly can be funny to no ends.
a master of this would be Christoffer!!
person 1- sir i dont think the bomb exploded
Person 2- what gave that away, the lack of noise? or the lack of mushroom? or the lack of the afterwave?
person3- it was the no mushroom for me.
person4- you don't say!!!
at an airport gate waiting to be let into the flight
Person 1- so dude we are going on gate number 15 rite?
person 2- oh yeah so lets join the queue for gate 15.
after reaching the near end of the line
person 3- i think we have reached our gate
person 1 and 2- You don't say!!
a mixture of sports quotations such as adidas with a hint of originality and innocence. it is a term that is described to take a wild action that should be done spontaneously and without using the mind. much similar to the creater of this phrase.
very commonly used by Leo and carry forwarded by Eetu and Rohan.
person1- dude do u think i should jump from that building to my death
person 2- don't think about it, you just do it.