"opportunity knocks" meaning in English

{
  "etymology_text": "From the proverb opportunity knocks at every man's door.",
  "head_templates": [
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        "2": "phrase"
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      "expansion": "opportunity knocks",
      "name": "head"
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  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "senses": [
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        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
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          "source": "w"
        },
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      "derived": [
        {
          "word": "when opportunity knocks"
        }
      ],
      "examples": [
        {
          "text": "Don't be caught napping When opportunity knocks.",
          "type": "example"
        },
        {
          "text": "Opportunity knocked in the form of an admission letter from the university.",
          "type": "example"
        }
      ],
      "glosses": [
        "An opportunity arises."
      ],
      "id": "en-opportunity_knocks-en-phrase-2v1WWUrG",
      "synonyms": [
        {
          "word": "opportunity comes knocking"
        }
      ]
    }
  ],
  "word": "opportunity knocks"
}

{
  "derived": [
    {
      "word": "when opportunity knocks"
    }
  ],
  "etymology_text": "From the proverb opportunity knocks at every man's door.",
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "phrase"
      },
      "expansion": "opportunity knocks",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English phrases",
        "English sentences",
        "English terms with usage examples",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "text": "Don't be caught napping When opportunity knocks.",
          "type": "example"
        },
        {
          "text": "Opportunity knocked in the form of an admission letter from the university.",
          "type": "example"
        }
      ],
      "glosses": [
        "An opportunity arises."
      ],
      "synonyms": [
        {
          "word": "opportunity comes knocking"
        }
      ]
    }
  ],
  "word": "opportunity knocks"
}