Semantically, I have discovered that the word "phlegm" is an ideal expletive. While it refers to a bodily secretion, it is not technically scatological. It manages to simultaneously avoid both the social taboos and the banality of more traditional vulgarities, eschewing the conventional form of "shock value" in favor of a genuine, visceral revulsion. When you utter "phlegm" as an expletive, the listener's mind immediately clicks onto the word's meaning.
The sound of the word is deeply evocative of its meaning, to the point of onomatopoeia. The dictionary insists that it's simply pronounced [flěm], but, personally, I tend to give "ph" a slightly more plosive emphasis than "f", and that "g" isn't so much "silent" as subliminal. [flěm] simply isn't as gutteral. With the proper pronunciation and emphasis, uttering "phlegm" at an irritating or offensive situation should clearly convey the impression that you are suppressing the impulse to clear your throat and spit.
It is my fervent** hope that, upon reading this, others will simply be too revolted to adopt this usage; if it were to enter common usage, it would soon lose the qualities that make it so effective an expression of displeasure, becoming as conventional as any of Carlin's Seven Words.
*Yes, this is how I communicate when I'm "incoherent". The little words go first, leaving a cascade of polysyllabic babbling that is only superficially erudite.
**in its literal meaning of "fevered"; rest assured that I would not wax so eloquently about such a topic were I, in fact, in my right mind.
***I suppose you could say that I'm "operating under the influenz-a".
****Or perhaps I'm merely a hack writer.