Totally and completely surprising, with the sense that whatever is described as such is shocking enough to change one's view of the world. However, the phrase is often used hyperbolically. Finding out that my dad isn't my biological father has been completely world shattering. I don't know who to trust anymore. A: "Oh my God, did you hear the world shattering news?" B: "Yeah, I can't believe that Sue is leaving the company. It won't be the same without her!"