"oh dark hundred" meaning in English

{
  "etymology_text": "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.",
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "oh dark hundred",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with topic categories using raw markup",
          "parents": [
            "Entries with topic categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English terms with non-redundant non-automated sortkeys",
          "parents": [
            "Terms with non-redundant non-automated sortkeys",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Times of day",
          "orig": "en:Times of day",
          "parents": [
            "Periodic occurrences",
            "Timekeeping",
            "Time",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "I had to get up at oh dark hundred to catch a plane.",
          "type": "example"
        }
      ],
      "glosses": [
        "Some unspecified hour in the early morning (implies an unpleasant time to be awake)"
      ],
      "id": "en-oh_dark_hundred-en-noun-JB01BeqN",
      "links": [
        [
          "hour",
          "hour"
        ],
        [
          "awake",
          "awake"
        ]
      ],
      "raw_glosses": [
        "(idiomatic) Some unspecified hour in the early morning (implies an unpleasant time to be awake)"
      ],
      "related": [
        {
          "word": "0-dark-thirty"
        },
        {
          "word": "dark thirty"
        },
        {
          "word": "oh dark thirty"
        },
        {
          "word": "zero dark thirty"
        }
      ],
      "synonyms": [
        {
          "word": "crack of dawn"
        },
        {
          "word": "stupid o'clock"
        },
        {
          "word": "wee small hours"
        },
        {
          "word": "early morning"
        },
        {
          "word": "0-dark-hundred"
        }
      ],
      "tags": [
        "idiomatic"
      ]
    }
  ],
  "word": "oh dark hundred"
}

{
  "etymology_text": "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.",
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "oh dark hundred",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "0-dark-thirty"
    },
    {
      "word": "dark thirty"
    },
    {
      "word": "oh dark thirty"
    },
    {
      "word": "zero dark thirty"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English entries with topic categories using raw markup",
        "English idioms",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English nouns with unknown or uncertain plurals",
        "English terms with non-redundant non-automated sortkeys",
        "English terms with usage examples",
        "en:Times of day"
      ],
      "examples": [
        {
          "text": "I had to get up at oh dark hundred to catch a plane.",
          "type": "example"
        }
      ],
      "glosses": [
        "Some unspecified hour in the early morning (implies an unpleasant time to be awake)"
      ],
      "links": [
        [
          "hour",
          "hour"
        ],
        [
          "awake",
          "awake"
        ]
      ],
      "raw_glosses": [
        "(idiomatic) Some unspecified hour in the early morning (implies an unpleasant time to be awake)"
      ],
      "tags": [
        "idiomatic"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "crack of dawn"
    },
    {
      "word": "stupid o'clock"
    },
    {
      "word": "wee small hours"
    },
    {
      "word": "early morning"
    },
    {
      "word": "0-dark-hundred"
    }
  ],
  "word": "oh dark hundred"
}