"fers" meaning in Ancien français

{
  "categories": [
    {
      "kind": "other",
      "name": "Formes d’adjectifs en ancien français",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Ancien français",
      "orig": "ancien français",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Ancien français",
  "lang_code": "fro",
  "pos": "adj",
  "pos_title": "Forme d’adjectif",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Wiktionnaire:Exemples manquants en ancien français",
          "parents": [],
          "source": "w"
        }
      ],
      "form_of": [
        {
          "word": "ferme"
        }
      ],
      "glosses": [
        "Cas sujet de ferme."
      ],
      "id": "fr-fers-fro-adj-J7iwJw9O"
    }
  ],
  "tags": [
    "form-of",
    "masculine"
  ],
  "word": "fers"
}

{
  "categories": [
    "Formes d’adjectifs en ancien français",
    "ancien français"
  ],
  "lang": "Ancien français",
  "lang_code": "fro",
  "pos": "adj",
  "pos_title": "Forme d’adjectif",
  "senses": [
    {
      "categories": [
        "Wiktionnaire:Exemples manquants en ancien français"
      ],
      "form_of": [
        {
          "word": "ferme"
        }
      ],
      "glosses": [
        "Cas sujet de ferme."
      ]
    }
  ],
  "tags": [
    "form-of",
    "masculine"
  ],
  "word": "fers"
}