One who thinks he/she is super great and attractive and must be idolized. Very selfish. Their head is so humongous that it cannot fit through a doorway. Comes on suddenly. Usually induced by alcohol. Includes a jumbo full teeth smile that is very phony. Lives a lifestyle which includes champagne wishes and caviar dreams on a beer budget. Always overshadows their mate. Is often seen as a family trait.
Can also be a verb: jackheadness
My husband is being a Jack Head
He was talking to that girl with his jackheadness