If someone is rambunctious, they are noisy, full of bounce, and hard to keep in a chair. We say it of children, puppies, a classroom after lunch, a party that has got out of hand. It is not usually cruel or criminal — more “too much energy to control” than “bad.” Informal, and more American than British (a British speaker might say boisterous or unruly instead). It can be fond: grandparents call the grandchildren rambunctious and smile. You would not normally use it for a quiet, cold kind of trouble. The word is a variant of older rumbustious, which already meant boisterous and unruly — same idea, slightly different shape. Nobody is quite sure why the vowels shifted; what students need is the feel: loud, lively, not sitting still.