The term 'hard head' can be described by two crossword solutions: OSTEO and SKULL. OSTEO is a five-letter word that refers to the hard, dense connective tissue that makes up the skeleton of vertebrates. SKULL is also a five-letter word that refers to the bony structure in the head of vertebrates that encloses and protects the brain. Together, these two words describe the term 'hard head, ' which is often used to refer to someone who is stubborn or unyielding.