v. To defriend; to remove from one's friends list (eg on a social networking website).
the GNU version of the Collaborative International Dictionary of English
n. One not a friend; an enemy.
The Century Dictionary and Cyclopedia
n. One not a friend; an enemy.
Word Usage
"I think it's no coincidence, though, that to "unfriend" - essentially dropping a connection with someone made on Facebook - is the Oxford American Dictionary's 2009 Word of the Year as this decade of frenzied communication comes to a close."