"slack up" meaning in English

{
  "forms": [
    {
      "form": "slacks up",
      "tags": [
        "present",
        "singular",
        "third-person"
      ]
    },
    {
      "form": "slacking up",
      "tags": [
        "participle",
        "present"
      ]
    },
    {
      "form": "slacked up",
      "tags": [
        "participle",
        "past"
      ]
    },
    {
      "form": "slacked up",
      "tags": [
        "past"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "*"
      },
      "expansion": "slack up (third-person singular simple present slacks up, present participle slacking up, simple past and past participle slacked up)",
      "name": "en-verb"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "verb",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English phrasal verbs with particle (up)",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "To slack; to let one's standards slip"
      ],
      "id": "en-slack_up-en-verb-HYFAmrSp",
      "links": [
        [
          "slack",
          "slack"
        ]
      ],
      "raw_glosses": [
        "(intransitive) To slack; to let one's standards slip"
      ],
      "tags": [
        "intransitive"
      ]
    }
  ],
  "word": "slack up"
}

{
  "forms": [
    {
      "form": "slacks up",
      "tags": [
        "present",
        "singular",
        "third-person"
      ]
    },
    {
      "form": "slacking up",
      "tags": [
        "participle",
        "present"
      ]
    },
    {
      "form": "slacked up",
      "tags": [
        "participle",
        "past"
      ]
    },
    {
      "form": "slacked up",
      "tags": [
        "past"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "*"
      },
      "expansion": "slack up (third-person singular simple present slacks up, present participle slacking up, simple past and past participle slacked up)",
      "name": "en-verb"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "verb",
  "senses": [
    {
      "categories": [
        "English entries with incorrect language header",
        "English intransitive verbs",
        "English lemmas",
        "English multiword terms",
        "English phrasal verbs",
        "English phrasal verbs with particle (up)",
        "English verbs"
      ],
      "glosses": [
        "To slack; to let one's standards slip"
      ],
      "links": [
        [
          "slack",
          "slack"
        ]
      ],
      "raw_glosses": [
        "(intransitive) To slack; to let one's standards slip"
      ],
      "tags": [
        "intransitive"
      ]
    }
  ],
  "word": "slack up"
}