In French, and very probably in the other Romance languages as well, the word "Lunatique" (lunatic) doesn't mean "crazy" at all.

Instead it means someone who is subject to frequent mood swings. (Like the moon).

The Latin-root words of the KJV are derived from French, so I would guess that this is the original meaning of the word and its sense has changed over time in English.

Since the KJV usage of the word clearly refers to a serious condition that Jesus "healed" I would guess that it refers to what we now call "bipolar disorder".