To be gobsmacked is to be very surprised or astonished. You'd probably be gobsmacked if an alien spaceship landed in your front yard.
Imagine how stunned you'd be if you were abruptly smacked in the mouth for no reason: This is where the word gobsmacked originated, from the Irish gob, or "mouth," and smack, or "hit." When you're gobsmacked, you're utterly shocked, overwhelmed, or taken aback. "When my dog was elected mayor of our town, we were all gobsmacked."