I'd heard people say it before, head cold. "I have a head cold," they'd say. Or, "Sorry, I can't come and play Nintendo with you, I've got a head cold."
My reaction would always be, "What the frik is a head cold?"
Well now I know what a head cold is because right now I have one.
It's kind of like a feeling where you just want to go home and bury your head in the couch cushions and drink tea in a wooly jumper, a wooly jumper that wraps around your head and keeps you warm and safe from the rest of the world. Nothing will penetrate your little woollen head fortress. And until you make your little woollen head fortress underneath the couch cushions you just feel chilly and vulnerable and tired and weak and a bit like a whiney whine annoying guy.
That's what a head cold is. It's also called winter.