"isin" meaning in Middle Irish

{
  "etymology_templates": [
    {
      "args": {
        "1": "mga",
        "2": "sga",
        "3": "isin"
      },
      "expansion": "Old Irish isin",
      "name": "inh"
    }
  ],
  "etymology_text": "From Old Irish isin.",
  "head_templates": [
    {
      "args": {
        "1": "mga",
        "2": "article form"
      },
      "expansion": "isin",
      "name": "head"
    }
  ],
  "lang": "Middle Irish",
  "lang_code": "mga",
  "pos": "article",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Middle Irish entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 9 entries",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "in the (accusative masculine/feminine singular)"
      ],
      "id": "en-isin-mga-article-AHuIL-He"
    }
  ],
  "word": "isin"
}

{
  "categories": [
    "Pages with 9 entries"
  ],
  "etymology_templates": [
    {
      "args": {
        "1": "mga",
        "2": "sga",
        "3": "isin"
      },
      "expansion": "Old Irish isin",
      "name": "inh"
    }
  ],
  "etymology_text": "From Old Irish isin.",
  "head_templates": [
    {
      "args": {
        "1": "mga",
        "2": "article form"
      },
      "expansion": "isin",
      "name": "head"
    }
  ],
  "lang": "Middle Irish",
  "lang_code": "mga",
  "pos": "article",
  "senses": [
    {
      "categories": [
        "Middle Irish article forms",
        "Middle Irish entries with incorrect language header",
        "Middle Irish non-lemma forms",
        "Middle Irish terms derived from Old Irish",
        "Middle Irish terms inherited from Old Irish",
        "Pages with 9 entries"
      ],
      "glosses": [
        "in the (accusative masculine/feminine singular)"
      ]
    }
  ],
  "word": "isin"
}