"oh dark thirty" meaning in English

{
  "etymology_text": "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.",
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "noun"
      },
      "expansion": "oh dark thirty",
      "name": "head"
    }
  ],
  "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": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "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 thirty 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_thirty-en-noun-Dz9QJC-8",
      "links": [
        [
          "hour",
          "hour"
        ],
        [
          "awake",
          "awake"
        ]
      ],
      "related": [
        {
          "word": "0-dark-hundred"
        },
        {
          "word": "oh dark hundred"
        },
        {
          "word": "dark thirty"
        }
      ],
      "synonyms": [
        {
          "word": "crack of dawn"
        },
        {
          "word": "stupid o'clock"
        },
        {
          "word": "wee small hours"
        },
        {
          "word": "early morning"
        },
        {
          "word": "0-dark-thirty"
        },
        {
          "word": "oh-dark-thirty"
        },
        {
          "word": "zero dark thirty"
        }
      ]
    }
  ],
  "sounds": [
    {
      "audio": "En-au-oh dark thirty.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4c/En-au-oh_dark_thirty.ogg/En-au-oh_dark_thirty.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/4/4c/En-au-oh_dark_thirty.ogg"
    }
  ],
  "word": "oh dark thirty"
}

{
  "etymology_text": "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.",
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "noun"
      },
      "expansion": "oh dark thirty",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "0-dark-hundred"
    },
    {
      "word": "oh dark hundred"
    },
    {
      "word": "dark thirty"
    }
  ],
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English idioms",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with usage examples",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Times of day"
      ],
      "examples": [
        {
          "text": "I had to get up at oh dark thirty 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"
        ]
      ]
    }
  ],
  "sounds": [
    {
      "audio": "En-au-oh dark thirty.ogg",
      "mp3_url": "https://upload.wikimedia.org/wikipedia/commons/transcoded/4/4c/En-au-oh_dark_thirty.ogg/En-au-oh_dark_thirty.ogg.mp3",
      "ogg_url": "https://upload.wikimedia.org/wikipedia/commons/4/4c/En-au-oh_dark_thirty.ogg"
    }
  ],
  "synonyms": [
    {
      "word": "crack of dawn"
    },
    {
      "word": "stupid o'clock"
    },
    {
      "word": "wee small hours"
    },
    {
      "word": "early morning"
    },
    {
      "word": "0-dark-thirty"
    },
    {
      "word": "oh-dark-thirty"
    },
    {
      "word": "zero dark thirty"
    }
  ],
  "word": "oh dark thirty"
}