I've always associated that sound with wagging fingers, although I've never heard it pronounced as clearly as 'tisk'. For me it's more of a 'clicking the tongue while dragging air through your teeth' kind of noise.

Also, these days (for me anyway) it has a slightly mocking, sarcastic tone to it. As in - someone's doing something mildly out of order, you both know it and you're pretending to give them a ticking off, but neither of you really care. It's not something I would ever do seriously.