Whether this particular gesture -- body language? Action? -- is unique to Greece or whether it is a broad Mediterranean phenomenon, I have never had a chance to discover; all I know is that most Greek people I know this side of the Atlantic do it rather than shake their heads to say, "No," or some equivalent thereof. My mother passed it on to my sister and me, though for a long time I resisted such ethnic influences and would snap at my sister, "You're Canadian! Stop trying to act so Greek!" whenever she would affect that mannerism.
The gesture itself is really quite simple. In its most basic form, is a lifting of the chin accompanied by raised eyebrows, half-closed eyes and a vocalized "tsk" sound. It seems so simple, yet those to whom I have described it can never really get it quite right, either pursing their lips so much that they make a sexual suggestion of the gesture or trying so hard to get the minutiae right that, ironically, they end up stiff-faced, when the very point of the gesture is to express. Some people add a negligent flip of the hand when brushing off a suggestion, others raise their gaze to the heavens, and still others close their eyes entirely. Some look you in the eye and just raise their eyebrows with that chin lift to indicate their fundamental disagreement. Still, the basic components remain the same, though each person adds his or her own touch to it -- usually her, as I have found that more women than men use this gesture. Whether it is a gender-specific one or not, I do not know; perhaps women use it more than men simply because Greek men are brought up to be less subtle.
This particular expression is a gentle negation similar to "I don't think so," only used when actually saying, "That's not true," or, "No," would be too confrontational. It is a step above an indifferent shrug and one below actually shaking one's head in subtlety, and the very fact that it is non-confrontational makes it that much harder to oppose in turn. For those who can master it, really make it their own, it is a versatile addition to anyone's physical vocabulary; for those who cannot, your best bet is to keep practicing in front of a mirror until you're no longer inadvertently offering people sexual favours.
Thanks to Apatrix, who has lived right in the culture in which this gesture was born, for some clarifications. Alas, my last visit to Greece eight years ago did not focus on body language, more's the pity.
TheLady says re Greek gestures of negation: The "tsk" sound, accompanied by a single shake of the head, or sometimes just a jerk or the head, and quite often closed or lowered eyes, is also quite widespread in Israel...Not just for negation, though, but for refusal, disapproval, lots of negative meanings. My teacher used to say the class sounded like a field full of cicadas when we were told to do something we didn't feel like doing...