"napkin math" meaning in English

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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "American English",
          "parents": [],
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          "kind": "other",
          "name": "Canadian English",
          "parents": [],
          "source": "w"
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        {
          "kind": "topical",
          "langcode": "en",
          "name": "Mathematics",
          "orig": "en:Mathematics",
          "parents": [
            "Formal sciences",
            "Sciences",
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            "Fundamental"
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          "source": "w"
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      "examples": [
        {
          "ref": "2018, Levi Lusko, I Declare War: Four Keys to Winning the Battle with Yourself, page 124:",
          "text": "Based on my napkin math—and keep in mind I am terrible at math—if you made every conscious choice to do right, your highest potential “life grade” is a D once your bad habits are subtracted.",
          "type": "quote"
        }
      ],
      "glosses": [
        "The act of performing quick mathematics often as a means of estimation, as if done on the back of a napkin (or in one's head)."
      ],
      "id": "en-napkin_math-en-noun-lT5eZN9T",
      "links": [
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          "Canada",
          "Canada"
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        [
          "US",
          "American English"
        ],
        [
          "mathematics",
          "mathematics"
        ],
        [
          "estimation",
          "estimation"
        ],
        [
          "napkin",
          "napkin"
        ]
      ],
      "raw_glosses": [
        "(Canada, US) The act of performing quick mathematics often as a means of estimation, as if done on the back of a napkin (or in one's head)."
      ],
      "related": [
        {
          "word": "back-of-the-envelope"
        }
      ],
      "tags": [
        "Canada",
        "US",
        "uncountable"
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    }
  ],
  "word": "napkin math"
}

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  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "back-of-the-envelope"
    }
  ],
  "senses": [
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        "American English",
        "Canadian English",
        "English entries with incorrect language header",
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        "en:Mathematics"
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      "examples": [
        {
          "ref": "2018, Levi Lusko, I Declare War: Four Keys to Winning the Battle with Yourself, page 124:",
          "text": "Based on my napkin math—and keep in mind I am terrible at math—if you made every conscious choice to do right, your highest potential “life grade” is a D once your bad habits are subtracted.",
          "type": "quote"
        }
      ],
      "glosses": [
        "The act of performing quick mathematics often as a means of estimation, as if done on the back of a napkin (or in one's head)."
      ],
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        [
          "US",
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          "mathematics",
          "mathematics"
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        [
          "estimation",
          "estimation"
        ],
        [
          "napkin",
          "napkin"
        ]
      ],
      "raw_glosses": [
        "(Canada, US) The act of performing quick mathematics often as a means of estimation, as if done on the back of a napkin (or in one's head)."
      ],
      "tags": [
        "Canada",
        "US",
        "uncountable"
      ]
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  ],
  "word": "napkin math"
}