It’s ghetto slang-used for someone w/ a big head whether through a medical condition or not—I heard it (not directed at me)back in Cleveland in the early 70s—hell, I read an OJ Simpson P/Boy interview from the 70s where he talked about kids teasing him by calling him “Waterhead” or “Headquarters” (he had a big head-literally)- as a kid.
Most of the time it sounds like “wa’ head” as in:
“I’m a kick that big ol’ wa’head %&*&%#@’s A$$....
Charming.