oh dark thirty

• 0-dark-thirty, oh-dark-thirty, zero dark thirty, dark thirty

From U.S. military terminology, in which hours before 10:00 am are prefixed with a zero, read as "oh", as in 0530, oh five-thirty, for 5:30 am.

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oh dark thirty

1. (idiomatic) Some unspecified hour in the early morning (implying an unpleasant time to be awake).

   Synonyms: crack of dawn, dark o’clock, stupid o'clock, wee small hours, see also Thesaurus:early morning

   I had to get up at oh dark thirty to catch a plane.