oh dark thirty

Etymology
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.

Noun

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

Synonyms

 * ,, ; see also Thesaurus:early morning