"foo fighter" meaning in English

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      "args": {
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        "2": "fighter",
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        "4": "type of aircraft"
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      "expansion": "fighter (“type of aircraft”)",
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  "etymology_text": "From foo, either a nonce word or a corruption of the French word feu (“fire”), + fighter (“type of aircraft”).",
  "forms": [
    {
      "form": "foo fighters",
      "tags": [
        "plural"
      ]
    }
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      "expansion": "foo fighter (plural foo fighters)",
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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
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        {
          "kind": "topical",
          "langcode": "en",
          "name": "Ufology",
          "orig": "en:Ufology",
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            "Celestial inhabitants",
            "Forteana",
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      "glosses": [
        "Any of various unidentified flying objects, as reported by Allied pilots during World War II."
      ],
      "id": "en-foo_fighter-en-noun-CToUvUle",
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          "unidentified flying object",
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          "pilot",
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          "World War II",
          "World War II"
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        "foo fighter"
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  "word": "foo fighter"
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      "args": {
        "1": "en",
        "2": "fighter",
        "3": "",
        "4": "type of aircraft"
      },
      "expansion": "fighter (“type of aircraft”)",
      "name": "m"
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  ],
  "etymology_text": "From foo, either a nonce word or a corruption of the French word feu (“fire”), + fighter (“type of aircraft”).",
  "forms": [
    {
      "form": "foo fighters",
      "tags": [
        "plural"
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