"dead on" meaning in English

{
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      "args": {
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      "expansion": "dead on (not comparable)",
      "name": "en-adj"
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  "lang": "English",
  "lang_code": "en",
  "pos": "adj",
  "senses": [
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      ],
      "examples": [
        {
          "text": "Her Sarah Palin impersonation is dead on!",
          "type": "example"
        }
      ],
      "glosses": [
        "Very accurate."
      ],
      "id": "en-dead_on-en-adj-pV9jUOe4",
      "links": [
        [
          "accurate",
          "accurate"
        ]
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      "tags": [
        "not-comparable"
      ]
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  "synonyms": [
    {
      "_dis1": "0 0",
      "word": "dead-on"
    }
  ],
  "word": "dead on"
}

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      "args": {
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      "expansion": "dead on (not comparable)",
      "name": "en-adv"
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  "lang": "English",
  "lang_code": "en",
  "pos": "adv",
  "senses": [
    {
      "categories": [],
      "examples": [
        {
          "text": "The train arrived dead on 2 o'clock.",
          "type": "example"
        }
      ],
      "glosses": [
        "Exactly at."
      ],
      "id": "en-dead_on-en-adv-4smEEX7I",
      "links": [
        [
          "Exactly",
          "exactly"
        ]
      ],
      "raw_glosses": [
        "(idiomatic) Exactly at."
      ],
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        "idiomatic",
        "not-comparable"
      ]
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  "synonyms": [
    {
      "_dis1": "0 0",
      "word": "dead-on"
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  ],
  "word": "dead on"
}

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  "lang": "English",
  "lang_code": "en",
  "pos": "adj",
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      ],
      "examples": [
        {
          "text": "Her Sarah Palin impersonation is dead on!",
          "type": "example"
        }
      ],
      "glosses": [
        "Very accurate."
      ],
      "links": [
        [
          "accurate",
          "accurate"
        ]
      ],
      "tags": [
        "not-comparable"
      ]
    }
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  "synonyms": [
    {
      "word": "dead-on"
    }
  ],
  "word": "dead on"
}

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  "categories": [
    "English adjectives",
    "English adverbs",
    "English entries with incorrect language header",
    "English lemmas",
    "English multiword terms",
    "English positive polarity items",
    "English uncomparable adjectives",
    "English uncomparable adverbs",
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  "head_templates": [
    {
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      "expansion": "dead on (not comparable)",
      "name": "en-adv"
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  "lang": "English",
  "lang_code": "en",
  "pos": "adv",
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      "examples": [
        {
          "text": "The train arrived dead on 2 o'clock.",
          "type": "example"
        }
      ],
      "glosses": [
        "Exactly at."
      ],
      "links": [
        [
          "Exactly",
          "exactly"
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        "(idiomatic) Exactly at."
      ],
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        "idiomatic",
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  "sounds": [
    {
      "audio": "en-au-dead on.ogg",
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  "synonyms": [
    {
      "word": "dead-on"
    }
  ],
  "word": "dead on"
}