"D-" meaning in Korean

{
  "etymology_templates": [
    {
      "args": {
        "1": "ko",
        "2": "en",
        "3": "-"
      },
      "expansion": "English",
      "name": "der"
    }
  ],
  "etymology_text": "From English D-Day.",
  "forms": [
    {
      "form": "D‐",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "di maineoseu",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ko",
        "2": "prefix",
        "head": "D‐",
        "tr": "di maineoseu"
      },
      "expansion": "D‐ • (di maineoseu)",
      "name": "head"
    }
  ],
  "lang": "Korean",
  "lang_code": "ko",
  "pos": "prefix",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Korean entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Korean terms in nonstandard scripts",
          "parents": [
            "Terms in nonstandard scripts",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "오늘은 여행 D‐2야.\nOneur-eun yeohaeng dimaineoseu‐i-ya.\nIt is 2 days before the trip today.",
          "type": "example"
        }
      ],
      "glosses": [
        "... days before the big event."
      ],
      "id": "en-D--ko-prefix-a4n9uCdl",
      "tags": [
        "morpheme"
      ]
    }
  ],
  "word": "D-"
}

{
  "etymology_templates": [
    {
      "args": {
        "1": "ko",
        "2": "en",
        "3": "-"
      },
      "expansion": "English",
      "name": "der"
    }
  ],
  "etymology_text": "From English D-Day.",
  "forms": [
    {
      "form": "D‐",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "di maineoseu",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ko",
        "2": "prefix",
        "head": "D‐",
        "tr": "di maineoseu"
      },
      "expansion": "D‐ • (di maineoseu)",
      "name": "head"
    }
  ],
  "lang": "Korean",
  "lang_code": "ko",
  "pos": "prefix",
  "senses": [
    {
      "categories": [
        "Korean entries with incorrect language header",
        "Korean lemmas",
        "Korean prefixes",
        "Korean terms derived from English",
        "Korean terms in nonstandard scripts",
        "Korean terms with usage examples"
      ],
      "examples": [
        {
          "text": "오늘은 여행 D‐2야.\nOneur-eun yeohaeng dimaineoseu‐i-ya.\nIt is 2 days before the trip today.",
          "type": "example"
        }
      ],
      "glosses": [
        "... days before the big event."
      ],
      "tags": [
        "morpheme"
      ]
    }
  ],
  "word": "D-"
}