"m/s/s" meaning in Conventions internationales

{
  "categories": [
    {
      "kind": "other",
      "name": "Symboles en conventions internationales",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Conventions internationales",
      "orig": "conventions internationales",
      "parents": [],
      "source": "w"
    }
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Lexique en conventions internationales de la métrologie",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Termes vieillis en conventions internationales",
          "parents": [],
          "source": "w"
        }
      ],
      "glosses": [
        "Symbole du mètre par seconde par seconde."
      ],
      "id": "fr-m/s/s-conv-symbol-EtmZyjOQ",
      "tags": [
        "dated"
      ],
      "topics": [
        "metrology"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "m/s²"
    }
  ],
  "word": "m/s/s"
}

{
  "categories": [
    "Symboles en conventions internationales",
    "conventions internationales"
  ],
  "lang": "Conventions internationales",
  "lang_code": "conv",
  "pos": "symbol",
  "pos_title": "Symbole",
  "senses": [
    {
      "categories": [
        "Lexique en conventions internationales de la métrologie",
        "Termes vieillis en conventions internationales"
      ],
      "glosses": [
        "Symbole du mètre par seconde par seconde."
      ],
      "tags": [
        "dated"
      ],
      "topics": [
        "metrology"
      ]
    }
  ],
  "synonyms": [
    {
      "word": "m/s²"
    }
  ],
  "word": "m/s/s"
}