Word-geekery alert

Good old English.

Consider -ard (or, occasionally, -art)*, what the Online Etymological Dictionary refers to as a “suffix of derogation.” Attached to an adjective or verb, it denotes someone who carries said action or quality to excess.

A bastard is base-born.

A braggart brags too much.

Before it became associated with a particular kind of bird, buzzard was a term for a species of raptor considered not good enough to hawk with.

A coward is easily cowed.

Though dastard now means “cad”, the word originally denoted an excessively stupid person, someone who was dazed.

A drunkard drinks (or is drunk) too much.

Stinkard needs no explanation.

Which brings us, of course, to wizard.

Indisputably, wizard was originally formed from wise + suffix of derogation.

There's one thing that we can safely say of the word wizard: root notwithstanding, this word was, at the time of its coinage, no term of compliment. A wizard must originally have been someone who was considered too wise for his own good.

I'll leave you to draw your own conclusions.

*The suffix -ard (Middle English via French via Germanic) itself derives from the adjective -hard, -hart, meaning “bold, hardy”, a suffix much-used in the formation of the old Common Germanic two-element name: Reginhart (“god-hardy”), Gerhart (“spear-hardy”), and Everard (“ever-bold”) among them.

If the semantic gap between “bold” and a derogatory suffix seems unbridgeably wide, consider the possibility that the usage may have arisen not so much from the word's intrinsic meaning as from its common use as a name-element: that drunkard, coward, and wizard may have originated as, in effect, mock names.