Thread:Sovq/@comment-4316832-20120323093434/@comment-1091639-20120325100414

Code to copy:
 * MediaWiki:Common.js - the part that starts with START OF TOOLTIP CODE
 * MediaWiki:Wikia.css - the part that starts with /* AJAX Tooltips */
 * The usage syntax can be found on my user page on that wiki - The one that says "AJAX Tooltip Caption"

Note that this tooltip requires the content to be displayed to be on another page thus might not be suitable for your needs. I can't offer more support than that since my JavaScript skills aren't exactly good either.