Meryn Stol voted up this answer. Joshua Engel The expression originally meant "I am being serious; this is the truth", dating at least back to the 1930s. It's a shortening of "no bullshit", and "bullshit" for something valueless or false is very old. The "that's obvious" use of it probably started in the 1960s (surprise surprise). It's a form of ironic exaggeration: if "no shit" means that something is the truth, then "no shit!" implies that it's obviously the truth. See question on Quora - Meryn Stol