"посерьёзневши" meaning in Russian

{
  "forms": [
    {
      "form": "poserʹjóznevši",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "past adverbial participle"
      },
      "expansion": "посерьёзневши • (poserʹjóznevši)",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "verb",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Russian entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "form_of": [
        {
          "extra": "poserʹjóznetʹ",
          "word": "посерьёзнеть"
        }
      ],
      "glosses": [
        "past adverbial perfective participle of посерьёзнеть (poserʹjóznetʹ)"
      ],
      "id": "en-посерьёзневши-ru-verb-5MARttys",
      "links": [
        [
          "посерьёзнеть",
          "посерьёзнеть#Russian"
        ]
      ],
      "tags": [
        "adverbial",
        "form-of",
        "participle",
        "past",
        "perfective"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[pəsʲɪˈrʲjɵzʲnʲɪfʂɨ]"
    }
  ],
  "word": "посерьёзневши"
}

{
  "forms": [
    {
      "form": "poserʹjóznevši",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "past adverbial participle"
      },
      "expansion": "посерьёзневши • (poserʹjóznevši)",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "verb",
  "senses": [
    {
      "categories": [
        "Pages with 1 entry",
        "Pages with entries",
        "Russian 5-syllable words",
        "Russian entries with incorrect language header",
        "Russian past adverbial participles",
        "Russian terms with IPA pronunciation"
      ],
      "form_of": [
        {
          "extra": "poserʹjóznetʹ",
          "word": "посерьёзнеть"
        }
      ],
      "glosses": [
        "past adverbial perfective participle of посерьёзнеть (poserʹjóznetʹ)"
      ],
      "links": [
        [
          "посерьёзнеть",
          "посерьёзнеть#Russian"
        ]
      ],
      "tags": [
        "adverbial",
        "form-of",
        "participle",
        "past",
        "perfective"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[pəsʲɪˈrʲjɵzʲnʲɪfʂɨ]"
    }
  ],
  "word": "посерьёзневши"
}