oh dark hundred
Definition from Wiktionary, the free dictionary
From military time, in which hours before 10:00 am are prefixed with a zero, read as "oh", and 00 minutes is read as "hundred", as in oh five-hundred for 5:00 am.
- (idiomatic) Some unspecified hour in the early morning (implies an unpleasant time to be awake)
- I had to get up at oh dark hundred to catch a plane.