어이 (or 어처구니) = a grip of millstone. 어이 없다 (or 어처구니 없다) = You can use it when something or a situation doesn't make sense in common way. for example when something that is supposed to be is absent. D: (similar emoticon)

"sense"요. 어이(어처구니) 없다 means something or someone has just left you dumbfounded because they're absurd or stupid, nonsense. Hope this helps. ^ ^