Grammatical explanation of "what the blank"

In emphatic questions, it's common to see or hear an interjection such as the heck — or something more vulgar — between the interrogative and the verb.

What was that?

becomes

What the heck was that?

Is there any way to interpret this grammatically, other than taking the heck as an interjection in its entirety? I guess it doesn't even qualify as an interjection, does it? I can’t think of a way you would parse the sentence properly. The problem is that it comes before the verb and not after. “What was the heck?”, although nonsensical, is grammatically simple. Bonus points if you can explain how this language feature evolved.

Solution 1:

A question like this got asked on alt.usage.english during the 90's, and for a while this answer to it was one of the most popular files on my website.

Solution 2:

Interesting question. In the case you give - What the heck was that? - then I would say the heck would be an adverb, modifying the verb was. It seems to be defined as such, with Wiktionary describing the heck as an:

Expletive used for emphasis after an interrogative word.

But bear in mind that such usages would be considered slang, in which case all bets are off as far as trying to pin them down grammatically speaking. Consider phrases such as:

What the heck?

Heck ya mean?

The hell I don't!

To me at least, the interpretation of these is not as clear.

Solution 3:

I once stumbled upon this website, transcribing a paper written about English Sentences Without Overt Grammatical Subjects, which whilst not covering "the heck" it does get across the difficulties of pinning down what these words are, and makes a good read of it.