Used in a way to respond to shock, something that caught you by surprise.
Also a short version of the spanish term hijo de la chingada.
40yr Old Virgin: Who are you talking to?
Superman: I'm talking to Bruce Lee's daughter, don't be jealous dude, I'm going over later and we're making babies.
(On the phone) Lee's Daughter: Illoo! Don't call me that.
(Overheard) Batman: Wow Superman, WOW.