Upon casting his creepy, yellowish eyes on his new wife, Edtard observes that she has already pealed her fucking wedding dress off. Edtard, being the flaming homosexual he is (he fucking SPARKLES and has never once dated a girl before Bella), sees her perky, high school girl breasts, and is suddenly disgusted.
Edtard: "Nyarg." ಠ_ಠ
Fortunately, he was able to get through the hot and sweaty night by thinking of a wild threesome between himself, Emmett, and Jasper as he violently pounded into his new wife, who, unbenownst to him, had a shattered pelvis and was suffering from internal bleeding.
It's a Breaking Dawn spoiler! Sorry, to any of you angry Twitards! I suppose I should have warned you earlier.
It can also be used when you have nothing else to say, because sometimes when you need a moment, you don't always have Twix.
Jane: "Dick...I-I have something to tell you. I've been thinking about it for awhile, and...well..."
Dick: "Yes? What is it, my sweet little bran muffin?"
Jane: "I...I love you, Dick!"
Additionally, 'nyarg' can be used when choking on the aforementioned bran muffin.
Dick: "Yeah, so, mmph...yum. Anyway, after looking at some rather delicious hentai, I--"
Carlos: "Hey, what's with the weird face? You...uh, you look like you're cumming. You okay, man?"
After Dick collapsed on the floor, twitching rather pathetically, Carlos continued to look on with abject fascination, completely oblivious to the growing tightness in his own pants. It was only after Dick's chest ceased to heave and his limbs went limp that Carlos realized that 'nyarg' was a call for help, and not, in fact, a lustful moan.
The bran muffin wasn't that particularly good, anyway.
Back to the point: That isn't to say that it couldn't be a lustful moan, though. It probably could be. But I don't think I have to provide an example for that one. You've probably got the idea by now.
Now, run off, little one! Share your knowledge of nyarg with the world! Remember that Mommy loves you!