"μτγν." meaning in Greek

{
  "forms": [
    {
      "form": "mtgn.",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "el",
        "2": "adjective"
      },
      "expansion": "μτγν. • (mtgn.)",
      "name": "head"
    }
  ],
  "lang": "Greek",
  "lang_code": "el",
  "pos": "adj",
  "senses": [
    {
      "alt_of": [
        {
          "extra": "(metagenésteri ellinikí): Koine Greek (the Greek used between c. 300 BC - 600 AD)",
          "word": "μεταγενέστερη ελληνική"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "Greek entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Abbreviation of μεταγενέστερη ελληνική (metagenésteri ellinikí): Koine Greek (the Greek used between c. 300 BC - 600 AD)"
      ],
      "id": "en-μτγν.-el-adj-v5wPoJVr",
      "links": [
        [
          "μεταγενέστερη",
          "μεταγενέστερη#Greek"
        ],
        [
          "ελληνική",
          "ελληνική#Greek"
        ],
        [
          "Koine",
          "Koine"
        ],
        [
          "Greek",
          "Greek"
        ]
      ],
      "tags": [
        "abbreviation",
        "alt-of"
      ]
    }
  ],
  "word": "μτγν."
}

{
  "forms": [
    {
      "form": "mtgn.",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "el",
        "2": "adjective"
      },
      "expansion": "μτγν. • (mtgn.)",
      "name": "head"
    }
  ],
  "lang": "Greek",
  "lang_code": "el",
  "pos": "adj",
  "senses": [
    {
      "alt_of": [
        {
          "extra": "(metagenésteri ellinikí): Koine Greek (the Greek used between c. 300 BC - 600 AD)",
          "word": "μεταγενέστερη ελληνική"
        }
      ],
      "categories": [
        "Greek abbreviations",
        "Greek adjectives",
        "Greek dictionary abbreviations",
        "Greek entries with incorrect language header",
        "Greek lemmas",
        "Greek terms spelled with ."
      ],
      "glosses": [
        "Abbreviation of μεταγενέστερη ελληνική (metagenésteri ellinikí): Koine Greek (the Greek used between c. 300 BC - 600 AD)"
      ],
      "links": [
        [
          "μεταγενέστερη",
          "μεταγενέστερη#Greek"
        ],
        [
          "ελληνική",
          "ελληνική#Greek"
        ],
        [
          "Koine",
          "Koine"
        ],
        [
          "Greek",
          "Greek"
        ]
      ],
      "tags": [
        "abbreviation",
        "alt-of"
      ]
    }
  ],
  "word": "μτγν."
}