The term 'Mandolin's Cousin' is a crossword solution that refers to the LUTE, a four-letter string instrument. The LUTE is a plucked string instrument with a neck and a deep round back, and is a member of the LUTE family. It is related to the mandolin, which is a stringed instrument that is similar in shape and size to the LUTE, but has a flat back and is usually tuned differently. The LUTE is believed to have originated in the Middle East and was popular during the Renaissance period.