An outspoken negative person who posts on internet forums under the misguided impression that people care about what they have to say. A pissface usually considers themself to be very important, but are generally mocked by those around them, including by friends and possibly even family.
A pissface is able to divert the course of a thread from happy and cheerful, to argumentative and angry by posting an inane and stupid comment that takes offense at an innocuous comment made by another poster.
On occassion, a thread moderator is a pissface, which usually leads to unnecessary wrangling of innocent and inoffensive posts made by others under the umbrella of "I thought that the post conflicted with the forum's rules". This is almost always a lie that the pissface states, to further cement their perceived power and self-importance. This leads to behind-the-back mocking by other wranglers, as they struggle to see how such a negative idiot could continue to be let away with the same antisocial behaviour time and time again.
You really suck all of the fun out of this forum,pissface.
Bob: "You wanna go online and abuse the pissface?"
Tim: "Yeah fuck yeah".
Someone (esp. on an Internet 'forum') who is actively negative, and sees the bad in everything. This doesn't necessarily reflect their public persona, but rather takes advantage of the faceless nature of the Internet to rant and rave at strangers without recourse. Similar to a troll, but not as antagonistic, pissfaces can also be called miserablists, pessimists or Grumpy Old Men.
They are often characterised by taking jocular comments at face value, finding offence where none was intended, and purveying a general attitude of negativity.
The term comes from the (supposed) 'sour' qualities of urine.
"Actually, that portion of the film is regarded as one of the finest in modern cinema. Before you see fit to criticize it like that, you should ask yourself what you *really* know about cinema. You disgust me"
"Please, don't be such a pissface"