A mutiny is a revolt, when the led no longer obey the leader. To be mutinous is to behave this way, like when the entire class rises up against a substitute teacher.
Mutinous behavior is largely associated with the behavior of unhappy crews at sea. A mutinous crew can really mess up a ship captain's day, but then again serve as a fantastic source of material for books and movies. There's something inspiring about mutinous behavior — it shows a certain defiant independent spirit that is admirable, unless of course you're the leader being mutinied against.