The opposite of "letting know" can be "keeping secret" or "withholding information." It means purposely not sharing information or telling someone something that they might want or need to know. Other antonyms for "letting know" can be "hiding," "concealing," or "covering up." These terms suggest some form of deception or discretion. Sometimes, it is necessary to keep something secret to protect someone or something, but it's important to communicate honestly and openly whenever possible. Keeping secrets can damage trust and relationships, so it's important to weigh the costs and benefits of withholding information before doing so.