Bone up is a phrase that means to STUDY or learn something in order to become more knowledgeable. The crossword solutions for this phrase are CRAM (4 letters), STUDY (5 letters), and OSSIFY (6 letters). Cramming is a method of studying where a person quickly reviews material in a short amount of time, usually before an exam. Studying is the act of learning and understanding information in order to gain knowledge. OSSIFY is a process where something hardens or becomes rigid, like bones. Therefore, the phrase bone up can be interpreted as studying or learning in order to become more knowledgeable, just like bones harden and become rigid.