Mustin is used when Justin Bieber looks ”musty”. But its important to add that he looks ”musty” because of his lyme disease. It affected his skin with acne and his mental health so that he did have the energy to take care of himself (like showering and eating properly). So using the term Mustin is mocking him with his disease.
”Justins hair looks greasy and look at his acne eww Mustin needs to shower”
Also used as a drag on stan twitter:
”Mustin tanked”