Jellyhead means: Nutcase. A person who is not straight in their head. Your shirt should always be tucked in to your trousers, which should reach half-way up your chest. The person most likely to tell you stories about when they “Nutted that bloke”, and what his assistant is doing with the money. There are many different conditions, and some people have worse bladder control than others. Jellyheads are known for shouting at cars passing by. Avoid. (in Community Dictionary, added by Keyon Simon)