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1.
To imitate in an extremely authentic manner various fart sounds by creating an airtight seal on your partner's neck...usually performed shortly after both parties have experienced an orgasm as a comic relief to the utter seriousness of the sexual act that just took place.

While this can be performed while not involved in sexual activity, it is usually more successful during sex as you are able to pick the most unexpected moment to strike.
Last night after several hours of sex with my girl, I decided to trip her up with a gigantic Neck Fart.

It sounded so lifelike and disgusting, you could almost taste the butt funk wafting through the air.
by Manson Family Opera April 18, 2009
 
2.
When someone is so overweight that their butt crack rises all the way up their spine to their back. This extended crack provides a channel for an anal fart to follow up thus seeming to come from the persons neck.
dude i was screwing this chubby chick from behind when i smelled something rank and that's when i realized she neck farted.
by ninjagoblin May 24, 2009