What is the prefix for sane?

The prefix for "sane" is "in-." When the prefix "in-" is added to "sane," it creates the Word "insane," which means not mentally sound or exhibiting deranged behavior. Prefixes are affixes added to the beginning of a Word to modify its meaning. In this case, the prefix "in-" changes the meaning of "sane" to its opposite, indicating a lack of sanity.