Used to describe physical discomfort experienced after drinking too much alcohol. He had such a thick head this morning he couldn't concentrate on anything.
Pro Tip 2/3
Colloquial Usage
Commonly used in casual, everyday conversation rather than formal writing or speech. Chatting with a friend, I mentioned I'd never again drink so much to end up with a thick head.
Pro Tip 3/3
Conversational Metaphor
Often employed metaphorically rather than literally to describe the hungover feeling. Feels like my brain is wrapped in cotton wool; I've got such a thick head today.