To drink large quantities of alcohol, to the extent that you do one, or both, of these two things:
a) Begin to speak in 'Queen's English'.
b) Reach ultimate rage and punch guys in the face.
Ah, man, I got totally Jennied last night! I ended up punching Seamus in the face!
To get completely and utterly pissed/fucked/razzed. So drunk that you find youself waking up in a room full of lebanese hookers the next day and not having a dime to pay their massive pimp.
Man - i got so jennied last night i have actually forgotten my own name!