"5th wheel" meaning in English

{
  "forms": [
    {
      "form": "5th wheels",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "5th wheel (plural 5th wheels)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "fifth wheel"
        }
      ],
      "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": "Five",
          "orig": "en:Five",
          "parents": [
            "Numbers",
            "All topics",
            "Terms by semantic function",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Alternative form of fifth wheel"
      ],
      "id": "en-5th_wheel-en-noun-Zd1UrDEz",
      "links": [
        [
          "fifth wheel",
          "fifth wheel#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "5th wheel"
}

{
  "forms": [
    {
      "form": "5th wheels",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "5th wheel (plural 5th wheels)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "fifth wheel"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms spelled with numbers",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Five"
      ],
      "glosses": [
        "Alternative form of fifth wheel"
      ],
      "links": [
        [
          "fifth wheel",
          "fifth wheel#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "5th wheel"
}