You must just

A conversational quirk where "must just" is used to suggest what someone should simply do. It adds a casual emphasis, implying that the solution or action is simple or obvious. Often used in giving advice or instruction in a friendly way.

A colloquial threat or warning structure, e.g. "You must just [do X]" with the rest implied. It actually means "You better not" or "You'd better watch out." It's often used as "You must just try me" (go on, dare to try – you'll regret it) or "He must just think I'm a fool" (he'd be mistaken to think that). The full consequence is left unsaid, making it an ominous ellipsis.