"fossil word" meaning in English

{
  "etymology_text": "Probably from the fact that a fossil was once a living organism, but is still of historical and scientific interest.",
  "forms": [
    {
      "form": "fossil words",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "fossil word (plural fossil words)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Linguistics",
          "orig": "en:Linguistics",
          "parents": [
            "Language",
            "Social sciences",
            "Communication",
            "Sciences",
            "Society",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "A word that is broadly obsolete but remains in current use because it is contained within an idiom that is still in use."
      ],
      "id": "en-fossil_word-en-noun-8jrEHgyI",
      "links": [
        [
          "linguistics",
          "linguistics"
        ],
        [
          "obsolete",
          "obsolete"
        ],
        [
          "idiom",
          "idiom"
        ]
      ],
      "raw_glosses": [
        "(linguistics) A word that is broadly obsolete but remains in current use because it is contained within an idiom that is still in use."
      ],
      "related": [
        {
          "word": "cranberry word"
        }
      ],
      "topics": [
        "human-sciences",
        "linguistics",
        "sciences"
      ]
    }
  ],
  "word": "fossil word"
}

{
  "etymology_text": "Probably from the fact that a fossil was once a living organism, but is still of historical and scientific interest.",
  "forms": [
    {
      "form": "fossil words",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "fossil word (plural fossil words)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "cranberry word"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Linguistics"
      ],
      "glosses": [
        "A word that is broadly obsolete but remains in current use because it is contained within an idiom that is still in use."
      ],
      "links": [
        [
          "linguistics",
          "linguistics"
        ],
        [
          "obsolete",
          "obsolete"
        ],
        [
          "idiom",
          "idiom"
        ]
      ],
      "raw_glosses": [
        "(linguistics) A word that is broadly obsolete but remains in current use because it is contained within an idiom that is still in use."
      ],
      "topics": [
        "human-sciences",
        "linguistics",
        "sciences"
      ]
    }
  ],
  "word": "fossil word"
}