Oxford senior lexicographer Christine Lindberg said the word had both currency and potential longevity.
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.
She said most prefixed words beginning with 'un' were adjectives, such as 'unacceptable' or 'unpleasant'.
"There are certainly some familiar 'un-' verbs (uncap, unpack), but 'unfriend' is different from the norm.
Facebook users often accept dozens, or even hundreds, of other users as “friends.”
Facebook users are also given the option of removing a Facebook friend. Removing a friend is call unfriending.
In fact, unfriending is easier than making a friend on Facebook. When making a friend, both members must agree to the “friendship.” However you can unfriend someone on Facebook without anyone else’s knowledge.
It can be done anonymously. No one else would know and the only way the unfriend would know is to look at their list of friends and see who’s name is no longer there.
Link