The term 'dead serious' is an idiom used to describe someone or something that is not joking and is completely serious. It is often used to emphasize the importance of a situation or to express the gravity of a situation. The crossword solution for this term is 'HUMORLESS', which means lacking in humor or wit. This accurately describes the seriousness of the situation, as someone who is dead serious would not be making jokes or light of the situation.