"mad hatter" meaning in English

{
  "etymology_text": "The term is attested since at least the 1790s, and a Mad Hatter character appears in Lewis Carroll's Alice's Adventures in Wonderland. See mad as a hatter.",
  "forms": [
    {
      "form": "mad hatters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "mad hatter (plural mad hatters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Fairy tale characters",
          "orig": "en:Fairy tale characters",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "langcode": "en",
          "name": "Lewis Carroll",
          "orig": "en:Lewis Carroll",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "A lunatic; a highly eccentric person."
      ],
      "id": "en-mad_hatter-en-noun-Cf8YZxhq",
      "links": [
        [
          "lunatic",
          "lunatic"
        ],
        [
          "eccentric",
          "eccentric"
        ]
      ],
      "related": [
        {
          "word": "March hare"
        }
      ],
      "wikipedia": [
        "mad hatter"
      ]
    }
  ],
  "word": "mad hatter"
}

{
  "etymology_text": "The term is attested since at least the 1790s, and a Mad Hatter character appears in Lewis Carroll's Alice's Adventures in Wonderland. See mad as a hatter.",
  "forms": [
    {
      "form": "mad hatters",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "mad hatter (plural mad hatters)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "March hare"
    }
  ],
  "senses": [
    {
      "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:Fairy tale characters",
        "en:Lewis Carroll"
      ],
      "glosses": [
        "A lunatic; a highly eccentric person."
      ],
      "links": [
        [
          "lunatic",
          "lunatic"
        ],
        [
          "eccentric",
          "eccentric"
        ]
      ],
      "wikipedia": [
        "mad hatter"
      ]
    }
  ],
  "word": "mad hatter"
}