oh dark thirty

• dark thirty
• oh-dark-thirty
• zero 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.

oh dark thirty

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

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

• crack of dawn
• stupid o'clock
• wee small hours