Examples of Janus words are:
These opposite meanings often occur because the word is, in fact, derived from quite different words. Cleave is an example of this: Cleave (verb) is descended from Middle English cleven, from Old English cleofan; akin to Old Norse kljufa meaning to split, Latin glubere meaning to peel, and Greek glyphein meaning to carve. On the other hand, cleave as an intransitive verb is descended from Middle English clevien, from Old English clifian, akin to Old High German kleben meaning to stick. Thus you have two very different sources for what appears today as a single word, resulting in a word that is its own antonym.
Luckily it's difficult to create a sentence using a Janus word that is truly ambiguous. Context will generally tip off the reader or listener to the intended meaning.
See if you can come up with an ambiguous sentence using one of the examples above. Can you think of any other Janus words? Post your replies!
Sources: Merriam-Webster's Eleventh Collegiate Dictionary and the Internet; logo from Janus Capital Group.