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1.
Taking back your approval of something that's been said or done, or something posted online.

This term originates from Facebook, where you can publicly say you "like" something, and then have the option to take the "like" back. "Unlike" does not mean you dislike something.
"I really like the shading and texture on this picture you drew of a man in a prison cell. Oh wait; the man is supposed to be me. Unlike."
by Moggraider July 29, 2009
 
2.
To reverse or negate one's previous liking of something. Unlike is, in that sense, the reciprocal of like. It does not mean "dislike." It is a word specifically reserved for negation of one's previous liking of something. It is not preemptive, thus a thing cannot be unliked before it has been liked. In order to be unliked, a thing must be currently liked, regardless of whether or not it has ever been liked or unliked in the past.
To put it mathematically:
If Like(7)=14, then Unlike(14)=7
but if Unlike(12)=5, then Like(5) does not necessarily equal 12.
He liked that she had gotten in the car. However, after the conversation turned for the mundane, he unliked it. He didn't mind it though, so he didn't dislike it.
by Count the Count January 05, 2010
 
3.
Origin: Button under Facebook status updates. Meaning: To change your mind quickly about something in an unfavorable way.
I liked the piece of art in the gallery at first, but then I took a closer look and instantly unliked it.
I thought I would buy that new dress, but when I saw the fabric up close I unliked it.
by yocarolyn October 22, 2009
 
4.
an expression used to indicate that you do not approve of whatever has just been said, or dislike it.
Originates from the ability to 'like' things on facebook and then 'unlike' them.
Guy 1: "Dude I just got the new Jonas Brothers album!"

Guy 2: "unlike..."
by shplurf March 17, 2009