A Janus word is a word that means both a thing (or an action) and its precise opposite.  The name is a reference to Janus, a Roman god with two faces.  If you didn’t catch that reference without it being explained, you probably aren’t very interested in this blog.

Some examples:

• Sanction
• Seed
• Weather

here are some other examples and longer discussions:

http://mentalfloss.com/article/49834/14-words-are-their-own-opposites

http://grammar.about.com/od/il/g/Januswordterm.htm