Kek literally translates to lol on World of Warcraft. When someone from the Horde side types lol in /say, members of the alliance side see kek instead. Not specific to Orcs.
Human Paladin: Dude, that Orc totally just pwned you.
Orc: Kek.
Human: Dude, now he's laughing at you. Kick his ass.
Not originally from World of Warcraft as some people think, 'kek' is the Korean approximation in english of laughter when typed on a Korean keyboard during Starcraft matches. As Starcraft was made by Blizzard as is world of warcraft, their connection to laughter in both games seems to be an homage of sorts.
Matt: OMGWTF you justgotzerg rushed in the face!!!11
Trevor:Fuck that!
Matt: kekekekekekekeke