Same as "crybaby" in League of Legends slang. Usually used to refer to someone who is complaining about something he considers unfair while the rest of the community thinks he's being a child.
Guy 1: OMG, you gotta nerf Eve now, she's so OP she killed my entire team in 3 bursts! I'm sick of this!
In Counter-Strike: Source, refers to hitting Q twice to switch between last selected weapon and current, usually to scope out faster then hitting right-click twice (like with AWPers and scouters do).
QQ is actually a counter strike noob action. Counter Strike noobs will randomly spam the "q" button which causes them to switch between their primary weapon and secondary pistiol and/or knife. The only time the QQ move is allowed is when someone is using an AWP (sniper rifle), this reduces the time from going into scope and also allows for the occasional no scope using the crosshair of your secondary as a placemarker. The reason why not to do QQ is simple, you are much more likely to be killed, when instead you sohuld be paying attention to your surroundings.