"miller's thumb" meaning in English

{
  "forms": [
    {
      "form": "miller's thumbs",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "miller's thumb (plural miller's thumbs)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English entries with topic categories using raw markup",
          "parents": [
            "Entries with topic categories using raw markup",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English terms with non-redundant non-automated sortkeys",
          "parents": [
            "Terms with non-redundant non-automated sortkeys",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "lifeform",
          "langcode": "en",
          "name": "Scorpaeniform fish",
          "orig": "en:Scorpaeniform fish",
          "parents": [
            "Fish",
            "Vertebrates",
            "Chordates",
            "Animals",
            "Lifeforms",
            "All topics",
            "Life",
            "Fundamental",
            "Nature"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "The European bullhead (“a species of fish, Cottus gobio”)."
      ],
      "id": "en-miller's_thumb-en-noun-e6VMFnGC",
      "links": [
        [
          "European bullhead",
          "European bullhead#English"
        ],
        [
          "species",
          "species"
        ],
        [
          "fish",
          "fish#Noun"
        ],
        [
          "Cottus gobio",
          "Cottus gobio"
        ]
      ]
    }
  ],
  "word": "miller's thumb"
}

{
  "forms": [
    {
      "form": "miller's thumbs",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "miller's thumb (plural miller's thumbs)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English entries with topic categories using raw markup",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with non-redundant non-automated sortkeys",
        "en:Scorpaeniform fish"
      ],
      "glosses": [
        "The European bullhead (“a species of fish, Cottus gobio”)."
      ],
      "links": [
        [
          "European bullhead",
          "European bullhead#English"
        ],
        [
          "species",
          "species"
        ],
        [
          "fish",
          "fish#Noun"
        ],
        [
          "Cottus gobio",
          "Cottus gobio"
        ]
      ]
    }
  ],
  "word": "miller's thumb"
}