Netspeak for "I love you, just kidding!" or, more commonly, "I love you, HA!, just kidding!", followed by a few lol's
Often used by girls trying to trick boys into thinking they like them, but using this is doing a bad job of it. A real prankster would put "ily", then "jk" about 30 seconds later, after the guy pours his soul.
Steve1990: "Yes, sometimes I just stare into the sunset wishing I could go back..."
Steve1990: *gasp* I do-- wait? what? did you just ilyjk me?
Sexy-Kaitlin: ily srsly