“Unfriend” – verb – To remove someone as a “friend” on a social networking site such as Facebook.
As in, “I decided to unfriend my roommate on Facebook after we had a fight.”
“It has both currency and potential longevity,” notes Christine Lindberg, Senior Lexicographer for Oxford’s US dictionary program. “In the online social networking context, its meaning is understood, so its adoption as a modern verb form makes this an interesting choice for Word of the Year. 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 real lex-appeal.”
(OUP Blog, Oxford University Press USA)