"duno-" meaning in Gaulois

{
  "categories": [
    {
      "kind": "other",
      "name": "Mots attestés en gaulois",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Préfixes en gaulois",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Gaulois",
      "orig": "gaulois",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Étymologies en gaulois incluant une reconstruction",
      "parents": [],
      "source": "w"
    }
  ],
  "etymology_texts": [
    "Dérivé d'une forme dunos, de l’indo-européen *dheuH-s- (« finir, achever, devenir un cercle fermé »)^([1])^([2]).",
    "Rapproché du vieil irlandais dún (fort, forteresse), du vieux breton din."
  ],
  "forms": [
    {
      "form": "-dunon"
    }
  ],
  "lang": "Gaulois",
  "lang_code": "gaulois",
  "pos": "prefix",
  "pos_title": "Préfixe",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Exemples en gaulois",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Exemples en gaulois à traduire",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "text": "Cantodunon."
        }
      ],
      "glosses": [
        "Fort, forteresse, citadelle, enceinte fortifiée."
      ],
      "id": "fr-duno--gaulois-prefix-2clg8naY"
    }
  ],
  "sounds": [
    {
      "ipa": "*\\dūno\\"
    }
  ],
  "tags": [
    "morpheme"
  ],
  "word": "duno-"
}

{
  "categories": [
    "Mots attestés en gaulois",
    "Préfixes en gaulois",
    "gaulois",
    "Étymologies en gaulois incluant une reconstruction"
  ],
  "etymology_texts": [
    "Dérivé d'une forme dunos, de l’indo-européen *dheuH-s- (« finir, achever, devenir un cercle fermé »)^([1])^([2]).",
    "Rapproché du vieil irlandais dún (fort, forteresse), du vieux breton din."
  ],
  "forms": [
    {
      "form": "-dunon"
    }
  ],
  "lang": "Gaulois",
  "lang_code": "gaulois",
  "pos": "prefix",
  "pos_title": "Préfixe",
  "senses": [
    {
      "categories": [
        "Exemples en gaulois",
        "Exemples en gaulois à traduire"
      ],
      "examples": [
        {
          "text": "Cantodunon."
        }
      ],
      "glosses": [
        "Fort, forteresse, citadelle, enceinte fortifiée."
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "*\\dūno\\"
    }
  ],
  "tags": [
    "morpheme"
  ],
  "word": "duno-"
}