mad as a hatter

A hatter is someone who used to make different types of hats, and they were often seen as being slightly off-kilter from everyone else and slightly crazy. This is where the phrase is supposed to originate from, though many people believe to be mad as a hatter is really to be mad as in angry or annoyed. Today, the phrase is usually pertaining to be mad as in crazy or mentally unstable, but often simply to act different from everyone else.