Abbreviation of very jealous. Most commonly used in Essex in the UK and became well known after the tv show 'The Only Way Is Essex' aired. sometimes even more abbeviated to just jel.
friend 1:"omg! I got all of Coldplays autographs!"
friend 2:"I'm well jel!"
'Well' is this case means 'very', 'jel' is an abreviaton for 'jealous' therefore 'well jel' means 'very jealous'
Person 1: How was your weekend?
Person 2: Great! I slept with Ben
Person 1: I'm well jel!