Webster's New World gives no etymology for crazy, but def 1 = having flaws or cracks. The entry above this is craze, which means to break or shatter from ME. It would appear crazy literally means cracked, a word we also like to use for crazy.

Sane, L. sanus or healthy. Insane is 'not sane,' that is, not healthy, although not necessarilly cracked.

Speaking of which, what of "inflammable," which has come to mean 'not inflammable?' (as well as the related "flammable")