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1. Having all the doors locked, with no ingress and egress permitted whatsoever. I'm sorry, sir, the building is locked down—no one is allowed in or out. The criminals ensured the building was locked down before they started opening the vault.
2. Of a computer or electronic device, put into a state of total protected from activation through passwords or encryption. The phone becomes locked down if you put in the wrong PIN more than three times. As soon as there is a security breach detected in the building, all the computer systems are automatically locked down.
3. In prison. What an animal—I hope he's locked down for the rest of his life. Unless you want to end up locked down like your former business partner, I suggest you start cooperating with us!
mod . [of a person] in jail. Mooshoo got himself locked down.