The phrase "lets slip" means to reveal or disclose something accidentally or unintentionally. Antonyms for this phrase would be to keep a secret, withheld information or stay silent. It would be the opposite of revealing or exposing something. Those who keep secrets and keep information to themselves withhold certain information for various reasons, while others feel it necessary to speak openly and transparently about everything. In any situation, there is always a choice to make. Whether to let information slip, or to keep it a secret is a personal decision, and it ultimately depends on the situation and the circumstances.