"Sent up river" is a colloquial expression that typically refers to sending someone to prison or other correctional institution. The antonyms, or opposite meanings, of this phrase would include "set free," "release," "acquit," or "pardon." These words imply that someone is not or will not be imprisoned or punished for their actions, instead being allowed to move forward relatively unencumbered. While "sent up river" can be used in a variety of contexts, including jokingly, these antonyms only apply specifically to legal proceedings or situations where a person might face penalties for their behavior.