the confining of prisoners to their cells, as following a riot or other disturbance.
a security measure taken during an emergency to prevent people from leaving or entering a building: The school remains under lockdown due to police activity in the area.

1970–75; lock1 + -down, probably extracted from nouns formed from phrasal verbs, such as crackdown, shutdown, etc.

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lockdown (ˈlɒkˌdəʊn)
(US) a security measure in which those inside a building such as a prison, school, or hospital are required to remain confined in it for a time: many schools remained under lockdown yesterday

Collins English Dictionary - Complete & Unabridged 10th Edition
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Dictionary.com's 21st Century Lexicon
Main Entry:  lockdown
Part of Speech:  n
Definition:  the state of being grounded and denied privileges
Usage:  slang
Dictionary.com's 21st Century Lexicon
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Word Origin & History

from 1940s in various mechanical senses, from lock (1) + down (adv.). Prison sense is by 1975, Amer.Eng.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The park will then go into lockdown, and every exit from the park will be
There is an intermediate school nearby that was in lockdown but that has been
  lifted and parents are getting their kids.
As news of the shooting broke, the campus shifted rapidly into lockdown mode.
The trains aren't running reliably and the streets are in a state of lockdown.
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