"long gray line" meaning in All languages combined

{
  "etymology_text": "After the gray winter overcoats worn by Army cadets.",
  "forms": [
    {
      "form": "the long gray line",
      "tags": [
        "canonical"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "def": "1"
      },
      "expansion": "the long gray line",
      "name": "en-prop"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "name",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "American English",
          "parents": [],
          "source": "w"
        },
        {
          "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": "Military",
          "orig": "en:Military",
          "parents": [
            "Society",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The graduates of the United States Military Academy."
      ],
      "id": "en-long_gray_line-en-name-2CjBg3B7",
      "links": [
        [
          "graduate",
          "graduate"
        ]
      ],
      "raw_glosses": [
        "(informal, US) The graduates of the United States Military Academy."
      ],
      "tags": [
        "US",
        "informal"
      ]
    }
  ],
  "word": "long gray line"
}

{
  "etymology_text": "After the gray winter overcoats worn by Army cadets.",
  "forms": [
    {
      "form": "the long gray line",
      "tags": [
        "canonical"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "def": "1"
      },
      "expansion": "the long gray line",
      "name": "en-prop"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "name",
  "senses": [
    {
      "categories": [
        "American English",
        "English entries with incorrect language header",
        "English informal terms",
        "English lemmas",
        "English multiword terms",
        "English proper nouns",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Military"
      ],
      "glosses": [
        "The graduates of the United States Military Academy."
      ],
      "links": [
        [
          "graduate",
          "graduate"
        ]
      ],
      "raw_glosses": [
        "(informal, US) The graduates of the United States Military Academy."
      ],
      "tags": [
        "US",
        "informal"
      ]
    }
  ],
  "word": "long gray line"
}