Edit in JSFiddle

JavaScript
HTML
CSS
Result

function showToolTip() {
  document.getElementById("tooltip").textContent = "I am tooltip text";
}

function hideToolTip() {
  document.getElementById("tooltip").textContent = "";
}

<body>
  <button disabled onmouseover="showToolTip()" onmouseout="hideToolTip()">
    I am disabled
  </button>
  <br>
  <span id="tooltip"></span>
</body>