"bye-letter" meaning in English

{
  "forms": [
    {
      "form": "bye-letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "bye-letter (plural bye-letters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "British 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"
        }
      ],
      "glosses": [
        "A postal letter carried only part of the way to the central post office, and delivered to its destination along the way."
      ],
      "id": "en-bye-letter-en-noun-5IckwvPM",
      "links": [
        [
          "postal",
          "postal"
        ],
        [
          "post office",
          "post office"
        ],
        [
          "deliver",
          "deliver"
        ],
        [
          "destination",
          "destination"
        ]
      ],
      "raw_glosses": [
        "(UK, historical) A postal letter carried only part of the way to the central post office, and delivered to its destination along the way."
      ],
      "related": [
        {
          "word": "bye-post"
        }
      ],
      "tags": [
        "UK",
        "historical"
      ]
    }
  ],
  "word": "bye-letter"
}

{
  "forms": [
    {
      "form": "bye-letters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "bye-letter (plural bye-letters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "bye-post"
    }
  ],
  "senses": [
    {
      "categories": [
        "British English",
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with historical senses",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "glosses": [
        "A postal letter carried only part of the way to the central post office, and delivered to its destination along the way."
      ],
      "links": [
        [
          "postal",
          "postal"
        ],
        [
          "post office",
          "post office"
        ],
        [
          "deliver",
          "deliver"
        ],
        [
          "destination",
          "destination"
        ]
      ],
      "raw_glosses": [
        "(UK, historical) A postal letter carried only part of the way to the central post office, and delivered to its destination along the way."
      ],
      "tags": [
        "UK",
        "historical"
      ]
    }
  ],
  "word": "bye-letter"
}