"back bone" meaning in English

{
  "forms": [
    {
      "form": "back bones",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "back bone (plural back bones)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "backbone"
        }
      ],
      "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": "Skeleton",
          "orig": "en:Skeleton",
          "parents": [
            "Body parts",
            "Body",
            "Anatomy",
            "All topics",
            "Biology",
            "Medicine",
            "Fundamental",
            "Sciences",
            "Healthcare",
            "Health"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Alternative form of backbone"
      ],
      "id": "en-back_bone-en-noun-suyVREni",
      "links": [
        [
          "backbone",
          "backbone#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "back bone"
}

{
  "forms": [
    {
      "form": "back bones",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "back bone (plural back bones)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "backbone"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Skeleton"
      ],
      "glosses": [
        "Alternative form of backbone"
      ],
      "links": [
        [
          "backbone",
          "backbone#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "back bone"
}