From Longman Dictionary of Contemporary Englishskullskull /skʌl/ ●●○ noun [countable] 1 
HBthe bones of a person’s or animal’s head2 → somebody can’t get it into their (thick) skull
HBthe bones of a person’s or animal’s head2 → somebody can’t get it into their (thick) skullExamples from the Corpus
skull• Firstly, it is reflected and given direction by the cut-away skull bone, acting as a parabolic reflector.• His bony skull and his sharp beak jutted out of the robe at such a low angle he looked like a buzzard.• He cuts up bleached gourds to be the skulls.• The superman testing himself with an ax, crushing the skulls of old women.• They are classified according to the number of openings in the skull.• He said post mortems showed Mrs Garvey had died from head injuries, including multiple fractures to the skull.• The skull smashed, apparently by a heavy mallet which Lorrimer had been examining.Origin skull (1200-1300) From a Scandinavian language 