This is an outdated slang term for being adept or skilled at something. You can be a whiz in the kitchen or a whiz at maths. Daniel Cassidy in his terrible book How The Irish Invented Slang, claimed that whiz in this sense comes from the Irish word uas, meaning outstanding, great, superior. Cassidy says that it is used as a prefix but apparently he didn’t understand what a prefix means. The word uas cannot stand on its own. It can only be used as a fossilised element in other words (like thuas, anuas, uasal) or as a prefix meaning maximum or upper. You can’t say that someone or something is uas at a particular activity. In other words, from the point of view of the Irish language, this claim is impossible and ridiculous.
In any case, there is a well-known and obvious English candidate for the source of this term. This is how Cassidy mentions and dismisses this candidate:
“Many dictionaries derive whiz from wizard. But, gee whiz, there is nothing magical about whiz. A whiz like Einstein got his genius from study, practice and hard work. Not metaphysics.”
Quite apart from the fact that metaphysics doesn’t mean ‘the supernatural’ (check out this book on Amazon on The Metaphysics of Relativity: http://www.amazon.com/Time-Metaphysics-Relativity-Philosophical-Studies/dp/0792366689), how is this an argument, even if it’s meant to be funny? You can say that someone is a wizard in the kitchen, but does that mean you think when they want to make a soufflé they don a cape and wave a wand around shouting Wingardium Leviosa? No, it means they’re good at what they do, just like the contracted form wiz.
Cassidy was such an idiot, I sometimes wonder whether he really had a degree at all, or if there is another Cornell somewhere (the Cornell Reformed Bible College, Jerk River, Kansas?) which might have awarded the Great Fraud a Mickey Mouse degree.