Someone who annoys you but you are still their friend.
Mel: No shes a piss face.
Abby: shes your best friend!!
Mel: i know, i just cant be bothered going.
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.
Bob: "You wanna go online and abuse the pissface?"
Tim: "Yeah fuck yeah".
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.
"Please, don't be such a pissface"