Abbreviation for the Japanese expression "wakuwaku tekateka" (わくわくてかてか), lit. "excited trembling/shaking".
Usually expressed before a certain event or occurrence (known or unknown to) the viewer/commenter is expecting to happen soon.
Commonly used on Japanese sites like the 2ch boards and Nico Nico Douga.
Often seen in conjunction with ktkr.
Case 1: Random video on Niconico where a random embarrassing/funny is expected to happen very soon. Often, people will comment with "ｗｋｔｋ", excited about what is going to happen.
Case 2: Random anime discussion thread on 2ch about previews of a random series. Usually, people will reply with "I really wonder what will happen wktk" "yeah wktk" "I'm so excited I can't wait/sleep wktk"
Case 3: Random boy ordered a random anime figurine, logs onto his random blog/facebook/twitter and goes "I ordered my 100th fig, should arrive some time next week wktk"