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on the hour

At every hour exactly; one o'clock, two o'clock, and so on. For example, The shuttle to New York departs on the hour. An extension of this idiom is every hour on the hour, meaning every time the clock's big hand reaches twelve, as in The bus passes by the house every hour on the hour.

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The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.