"it's on" meaning in English

{
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      "args": {
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      "expansion": "it's on",
      "name": "head"
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  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "senses": [
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          "kind": "other",
          "name": "English entries with language name categories using raw markup",
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          "source": "w"
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      "derived": [
        {
          "word": "on like Donkey Kong"
        }
      ],
      "examples": [
        {
          "ref": "1959: Girls Town (film)",
          "text": "[A man from a competing gang throws a drink in Fred's face, just before a fight begins.]\nFred Alger (Mel Tormé): It's on!"
        }
      ],
      "glosses": [
        "A conflict is starting."
      ],
      "id": "en-it's_on-en-phrase-JDI60984",
      "links": [
        [
          "conflict",
          "conflict"
        ]
      ],
      "raw_glosses": [
        "(colloquial) A conflict is starting."
      ],
      "related": [
        {
          "word": "bring it on"
        }
      ],
      "tags": [
        "colloquial"
      ]
    }
  ],
  "word": "it's on"
}

{
  "derived": [
    {
      "word": "on like Donkey Kong"
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "phrase"
      },
      "expansion": "it's on",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "related": [
    {
      "word": "bring it on"
    }
  ],
  "senses": [
    {
      "categories": [
        "English colloquialisms",
        "English entries with language name categories using raw markup",
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      "examples": [
        {
          "ref": "1959: Girls Town (film)",
          "text": "[A man from a competing gang throws a drink in Fred's face, just before a fight begins.]\nFred Alger (Mel Tormé): It's on!"
        }
      ],
      "glosses": [
        "A conflict is starting."
      ],
      "links": [
        [
          "conflict",
          "conflict"
        ]
      ],
      "raw_glosses": [
        "(colloquial) A conflict is starting."
      ],
      "tags": [
        "colloquial"
      ]
    }
  ],
  "word": "it's on"
}