A derogatory term for drug-users at dance clubs. Most commonly employed when describing Methylenedioxymethamphetamine (Ecstasy) users. The term itself comes from the fluid head movements done by those described; appearing as though their neck is made of jelly.
"Yes, Jason and I used to hit the clubs all the time, but we have not since he became a jellyneck."
a jelly type of neck
hannah had a very sort of jellyneck