“We don’t like the US government very much. Their boats are weak, their lulz are low, and their sites aren’t very secure. In an attempt to help them fix their issues, we’ve decided to donate additional lulz in the form of owning them some more! This is a small, just-for-kicks release of some internal data from Senate.gov – is this an act of war, gentlemen? Problem? – Lulz Security.”
Lulz is a derivative of lol (laugh out loud) that has evolved to mean doing something because you can and you get satisfaction out of it -Ed.