Green’s Dictionary of Slang

(N.Z.) a fool.

1992		R. Park Fence Around the Cuckoo 136: ‘A gang of jellyheads,’ said Mera.
2003		McGill Reed Dict. of N.Z. Sl. 85: A gellyhead/jellyhead was initially somebody wobbly from too much inadvertent inhalation of gelignite fumes, broadened to mean an idiot.