New Oxford American Dictionary 2009 Word of the Year is ‘’unfriend’’
The meaning of the word is to remove someone as a ‘friend’ on a social networking site such as Facebook. Example, “I decided to unfriend my roommate on Facebook after we had a fight.”
OUP Blog gives more reasons for choosing this word.
‘’Most “un-” prefixed words are adjectives (unacceptable, unpleasant), and there are certainly some familiar “un-” verbs (uncap, unpack), but “unfriend” is different from the norm. It assumes a verb sense of “friend” that is really not used (at least not since maybe the 17th century!). Unfriend has ‘’ currency and potential longevity.”
Have a look at the list of words that were considered along with ‘’Unfriendly’’ for the New Oxford American Dictionary 2009 Word of the Year.