Cotangent

Cotangent: noun. A trail of thought, which is shared by two or more people and isn't pertinent to the conversation which was held previously, or has side-tracked by continuing the conversation for too long. Cotagenting developed from prolonged exposure to other people during the age of letters. The people of Britain and France in the 18th century were uncertain how to speak since they were used to conversing through letters. In letters they had time to think through what they wanted to say, but when they were placed in the spotlight face to face, they had no time to think through everything carefully and would often go together on cotangents.