Letting out refers to releasing something or someone. The opposite of letting out can be keeping in, restraining, confining, withholding, or imprisoning, depending on the context. Keeping in can be used when referring to emotions, secrets, or confidential information. Restraining or confining is the opposite of letting out someone or something. Withholding refers to keeping something back intentionally. Imprisoning someone is the ultimate act of not letting them out. The opposite of letting out can be used in different contexts, such as in expressions, phrases, or sentences. Choosing the right antonym depends on the intended meaning of the sentence or context in which it is used.