The phrase "jaw dropper" means something that is extremely surprising or shocking, to the point that it makes your mouth fall open in surprise. Full definition
The biggest jaw dropper for us all; me, the mister, good friends # 1 and # 2, and the owners was this: The gent that lived on the second floor directly above my friend was a hoarder. (acolourfulcanvas.com)
... Although the video as a whole is clearly designed to sell the game play (which looks great), the real jaw dropper of this latest video entry is the first clip form a documentary titled The Goblin Man of Norway. (que-ee.com)
One of the biggest jaw droppers at E3 for me last year was Far Cry 3. (jgghgames.com)