Back in the early sixties one of those mad, mad men decided there was a need for a punctuation mark that expressed both a question and extreme surprise. We think this must be something like that television ad for the rental car company where John McEnroe is told he can choose any car on the lot and he exclaims in tone of outraged surprise: “Choose any car? You cannot be serious?”

Advertising executive Martin K. Speckter thought there ought to be a punctuation mark for print ads that expressed the combination of extreme surprise and rhetorical question that McEnroe does so beautifully with his voice. So Speckter came up with what came to be known as the interrobang, a combination of the exclamation mark and question mark.
Ever heard of it? Probably not. While it was something of a fad in the sixties, it never caught on as standard punctuation. Although Microsoft includes several versions in its Wingdings II character set, the interrobang is not on the keyboard along with the common question mark. We can’t use it in the text here, only as a picture.

So, the relevance. Writers do seem to have a need to combine question and exclamation marks. We see this all the time as in:

“You cannot be serious?!” Or, “You cannot be serious!?”

Both ways, because we ran across an Internet query asking which was correct: !? or ?! The answer is neither, if you want to be taken seriously as a writer. This is not correct punctuation. If you want to express in writing what McEnroe does with his voice, write about what his indignation expresses, that he has been mistreated by car rental companies everywhere for a long time and cannot believe that one of them could possibly let him choose his own car. Tell the story, don’t circumvent it with punctuation. The interrobang is not an option.