Presh is a word describing someone who is irritable, sulky, upset, moody or simply angry. There are many ways to become presh. One is either by denying the accusation of being presh, even if one was not presh to begin with. One is to ignore japes or non-chalantly say "Good one, buddy" in a sarcastic manner. People who are presh usually ignore the banter around them and sulk to themselves, often exclaiming "I just want to do my work!". They will strongly deny their presh by claiming "I'm not presh, why would I be presh?"
The accuracy of presh can be measured by the Richter presh scale, invented by Dr. Richter who famously said "No, you wouldn't understand you're a dick because you're a cunt!"
Ian: No, how am I presh?
Jared: I'd say Brychan's a 9.3 on the Richter presh.