Rapper MOS is a three-letter acronym for 'Master of Speech, ' which is a title given to a hip-hop artist who has achieved a high level of lyrical skill and creativity. This title is often used to describe the most successful and influential rappers in the industry, such as Jay-Z, Eminem, and Kanye West. The term is also used to refer to any rapper who has achieved a certain level of success and recognition in the rap music scene.