Stinkface

[Sonia]

when you rub your ass in someones face leaving them disgusted. Started out as a wrestling move used by the Rikishi.

[Hina Muqadas]

the art of planting smelly,freshly crapped ass upon someones face and shimmying to and fro-(causing a poop smear on the victims face)

[PussyCat]

Many people define the stinkface to be when one sticks their ass in anothers face, leaving their face smeared in crap. This definition denounces those other definition. The stinkface originated as a wrestling move; since this move does not cause any injuries, it is used mostly to embarass the victim.
Just to establish the definition of a stinkface, it is a wrestling move that (usually) involves two people. In classic wrestling, one opponent is thrown against one of the poles, and sags to the ground, defeated. The other opponent then backs their ass into the victims face, rubbing the fat of their ass around and smothering the victim.
The stinkface has many uses. Lets explore those:

Humiliation: The original purpose. To humiliate an opponent rather than injure them.

Punishment: To some people, receiving a stinkface is not pleasurable, so this move can be used as a consequence.

Above, it was mentioned that stinkfaces can include more than one person. This option relates to Use No. 2: Punishment. The victim is obviously reluctant to receive the stinkface, so sometime another person is needed to restrain the victim.
Types of stinkfaces:

Regular: The victim sags against the pole (or wall) and the stinkfacer backs his/her ass into the victims face and grinds.

Against-the-Wall: The victim sits up straight against a wall and is smothered with ass when the stinkfacer closes in. This is also a type of [facesitting].

Man-to-Man Stinkfacing: When a man stinkfaces another man, this will usually be a prank.
Man-to-Woman Stinkfacing: This does not happen a lot, but is probably a prank as well.
Woman-to-Man Stinkfacing: This will most likely be wrestling humiliation, or part of femdom practices.
Woman-to-Woman Stinkfacing: Part of wrestling humilation.
When men do stinkfaces, they wear pants.
When woman do stinkfaces, they wear thongs or lingerie.
Stinkfaces are simple enough to understand, with this handy definition.

[Mark Zuckerberg]

rather a distasteful action, planting ones [ass] in someone elses face, and rubbing it in, therefore leaving their face smeared in [shit], and smelling like whatever you had eaten that day.

[Tanya Shivari]

The act of losing a bet and having to endure smelling your friends butt and smell their farts that go up your nose.

[Jason Yan]

The face one makes when a musician or music touches their soul. That a musicians talent and soul would put ones face in a cringe as if they smelt something funny or stinky.