To have a really big head, usually made more obvious by a small body or neck.
Also can be related to Sheen family of actors (Martin, Charlie and Emilio Estevez), who have dense foreheads.
"Man that Joe Buck's got some head sheen."
"Rihanna's hot, but boy does she have some sheen on her head."
"I wanted to borrow Joe's hat, but it was too tight because I've got too much fucking headsheen."