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Phrase(s): at the top of the hour
Fig. at the exact beginning of an hour. (Alludes to the big hand on a clock pointing to the 12. Often heard on television or the radio. See also at the bottom of the hour.) • Every class in my school starts at the top of the hour. • Our next newscast will be at the top of the hour.
McGrawhill's American Idioms And Phrasal Verbs