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let (something) slip
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.
let something slip (out)
Fig. to reveal a secret carelessly or by accident. I didn't let it slip out on purpose. It was an accident. John let the plans slip when he was talking to Bill.
1. Also, let slip or slide by ; let slide. Miss an opportunity; waste time. For example, We forgot to buy a ticket and let our big chance slip by, or He let the whole day slide by. The first term dates from the mid-1500s, the variant from the late 1500s.
2. Also, let slip out. Reveal something, usually inadvertently, as in He let it slip out that he had applied for the vacant position. [Mid-1800s]
3. let slip through one's fingers. Fail to seize an opportunity, as in We could have won the trophy but we let it slip through our fingers. [First half of 1600s]
To say inadvertently.