Comrade-in-arms is a term used to describe two or more people who are united in a common cause, such as a battle or a struggle. An ALLY is someone who is associated with another person or group in a relationship of mutual support and understanding. The ULNA is one of the two bones in the forearm, located on the medial side of the arm. Together, these two words form the term comrade-in-arms, which is used to describe two or more people who are united in a common cause.