As those who know me slowly get to realize, I'm something of a grammar freak. I read Eats, Shoots & Leaves for fun and actually enjoyed it (and found a mistake in it; not a typo, but a minor substance error), despite the lack of the Oxford comma in the title. On that note, I have strong feelings about the Oxford comma. (I'm for it. Strongly.)
Something about how grammar imparts structure and organization to language appeals to the engineer in me. (Ironically, surrounding my computer right now is somewhat of a holy mess of papers and other desk-related paraphernalia. I'd like to organize it, but the lazy-guy-in-me beats the engineer-in-me almost every time.)
With any good organizational system, everything has a function. Even better, when analyzed methodically, gaps, if any, in the system are unmistakable. I believe I have found one.
The interjection is used to express an emotion or sentiment on the part of the speaker. Syntactically, it's usually used by itself or at the beginning of a sentence. There are other parts of speech which may be used to give an indication of something about the speaker, including the ejaculation (which almost always stands on its own), the discourse marker, and the filler.
There is another way that words are used to express the emotion or sentiment of the speaker, but isn't, to my knowledge, recognized as an interjection. This is because it's hiding as an adjective. It comes before a noun or noun phrase, but doesn't modify it at all. Rather, it's used by the speaker to express an emotion, usually a negative one. Thus, it deserves its own, new part of speech.
What do I mean? I'm glad you #*@%-ing asked. Here's an example: You're eating dinner, and ask someone if he can pass you the rice. He says he can. You ask him if he will pass you the rice, he cleverly (to his mind, at least) says that he might if you ask. You then tell him to "pass the #*@% ing rice". The word preceding "rice", syntactically, is being used as an adjective would. However, it in no way is being used to describe the rice (if it is, I'd like that recipe), but as a reflection on your emotional state, as the speaker, toward your dinner-mate. In this case, it's justifiable, maybe borderline-homicidal, frustration.
(A neighbor of mine pointed out that sometimes it actually is an adjective, as in "damned rice", the implication being that you're taking you're frustration out on the food and expressing your desire for it to spend an eternity on Satan's buffet. I'll concede that sometimes this could be the case, but generally speaking, that's generally not true. Most times, other words are used, which indicate a gory state, as in the British "bloody", copulation, or nothing at all, as in "frickin'".)
Since it's somewhat of a cross between an interjection and an adjective, having the properties of the former, but being used syntactically as the latter is, I've come up with the portmanteau "interjective" for this new part of speech. Spread the word. It's gonna be a thing. A #*@%-ing thing.