The proper response to the exclamation “All hail the Interrobang!” is “What the?!” That should be followed, of course, by an “Oh yeah, that thing…” as soon as you’re reminded of what heck an interrobang is.
The interrobang is a modern but non-standard punctuation mark. It combines a question mark (?) with an exclamation mark (!) in a variety of fashions. (?! and !? are the most obvious and common.)
You use an interrobang to show exasperation, or befuddlement. It accompanies rhetorical questions driving by frustration. You’re not really asking a question, but you wish there was an answer. If you remember Mango from SNL, Mango’s “What the Frick?!” ends with an interrobang.
I have a love/hate relationship with interrobangs. On the one hand, they are lively, serve a purpose, and remind us that language is ever evolving. On the other hand, I usually encounter them used by lazy writers who lump two punctuation marks together (or four ?!?!) because they can’t be bothered to figure out what they really mean to write.
Time will judge the interrobang, as it will judge us all. For now, all hail the interrobang as the sign of our times.