Open to argument' means that someone is willing to listen to and consider other people's opinions or ideas. The term 'MOOT' is used to describe something that is open to debate or discussion. It comes from the Old English word 'MOT, ' which means 'meeting for discussion.' Therefore, the crossword solution 'MOOT' for the term 'open to argument' means that someone is willing to engage in debate or discussion about a topic.