(phrasal verb) to stand someone up; not turning up for a date or appointment; to break a promise; to fail to so something as promised; to have a secret exposed; to mess up something; to have the base (usually of a container) falling off.
describes a person (usually a child) who has regained freedom after being restrained and is now acting up; literally 'a monkey whose rope has been untangled '; someone who is unruly or out of control