a thick head
1. An inability to retain information or understand instructions. He repeated the directions twice, but I just couldn't get it through my thick head.
2. A sore head and unclear thoughts resulting from being sick or having drunk too much alcohol the night before. I couldn't concentrate because of the thick head my sinus infection was giving me. We all came in with thick heads the day after the staff Christmas party, so no one was really working too hard.
3. Of beer, a thick, rich foam produced upon being poured into a glass. I've personally never understood the appeal of drinking beers with thick heads. It just seems like a waste of space in the glass to me.