To accidentally or inadvertently reveal some secret or important piece of information. I can't believe your sister let news of our engagement slip in front of your parents! The agent, under the influence of a powerful truth serum, let slip the names of other operatives working covertly in the area.

To accidentally or inadvertently reveal a given secret or important piece of information. We had done so well keeping our engagement a secret. I can't believe I let it slip in the middle of dinner last night!