Zero hour (http://www.usingenglish.com/reference/idioms/zero+hour.html)
The time when something important is to begin is zero hour.
Zero tolerance (http://www.usingenglish.com/reference/idioms/zero+tolerance.html)
If the police have a zero tolerance policy, they will not overlook any crime, no matter how small or trivial.
Zigged before you zagged (http://www.usingenglish.com/reference/idioms/zigged+before+you+zagged.html)
If you did things in the wrong order, you zigged before you zagged.
Zip it (http://www.usingenglish.com/reference/idioms/zip+it.html)
This is used to tell someone to be quiet.
Zip your lip (http://www.usingenglish.com/reference/idioms/zip+your+lip.html)
If someone tells you to zip your lip, they want to to shut up or keep quiet about something. ('Zip it' is also used.)

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