1. An attempt to quash or suppress something, especially the spread of rumors or sensitive information. We need to keep the lid on rumors about the company going bankrupt. A: "But boss, what if the police find out about the money laundering?" B: Put a lid on it, Tom. They won't find out anything so long as everyone keeps their mouths shut."

2. An especially negative conclusion to an already bad experience or situation. Sometimes worded as "tin lid" in this usage. The downgrading of the company's credit rating puts the lid on a disastrous year for their financial situation. What a bad day: I left my phone in the grocery store, my car broke down on the way home, and, to put the tin lid on it, my groceries broke through the bag as I was walking to the front door.