"задницей" meaning in Russian

{
  "forms": [
    {
      "form": "за́дницей",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "zádnicej",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "noun form",
        "g": "f-in",
        "head": "за́дницей"
      },
      "expansion": "за́дницей • (zádnicej) f inan",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Russian entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "form_of": [
        {
          "extra": "zádnica",
          "word": "за́дница"
        }
      ],
      "glosses": [
        "instrumental singular of за́дница (zádnica)"
      ],
      "id": "en-задницей-ru-noun-kkJ9ro5h",
      "links": [
        [
          "за́дница",
          "задница#Russian"
        ]
      ],
      "tags": [
        "form-of",
        "instrumental",
        "singular"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[ˈzadʲnʲɪt͡sɨj]"
    }
  ],
  "word": "задницей"
}

{
  "forms": [
    {
      "form": "за́дницей",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "zádnicej",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "noun form",
        "g": "f-in",
        "head": "за́дницей"
      },
      "expansion": "за́дницей • (zádnicej) f inan",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "Pages with 1 entry",
        "Pages with entries",
        "Russian 3-syllable words",
        "Russian entries with incorrect language header",
        "Russian non-lemma forms",
        "Russian noun forms",
        "Russian terms with IPA pronunciation"
      ],
      "form_of": [
        {
          "extra": "zádnica",
          "word": "за́дница"
        }
      ],
      "glosses": [
        "instrumental singular of за́дница (zádnica)"
      ],
      "links": [
        [
          "за́дница",
          "задница#Russian"
        ]
      ],
      "tags": [
        "form-of",
        "instrumental",
        "singular"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[ˈzadʲnʲɪt͡sɨj]"
    }
  ],
  "word": "задницей"
}