The worst insult anyone could call you. It's meaning is something unspeakable, even in this eternal hellhole that is the internet. It's not even considered a slur, which only makes it worse, as the person saying it doesn't even consider you worth enough to sacrifice their profesionality just to insult you.
Dublin knackersmating call to each other, often let out in a loud screach, much like a seagull. Often used as a back up call to fellow inmates or 'budz' when starting fights or 'scraps'. When used by professionals can only be heard by members of the same herd or 'gang' as it is too highpitched to be heard by regular ears (akin to dog whistles).