oh dark hundred

Etymology
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.

Noun

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

Synonyms

 * ,, ; see also Thesaurus:early morning