It's a short form of a word meaning 'tut, tut'. It's from "쯧쯧" -> "ㅉㅉ" it sounds similarly so people. made it short.

But you would be better not to use that words. If you express that feeling then '아...(means I understand this situation) ' is better. I think ㅉㅉ is rude somewhat. It's common used of some of very young people still other young people don't use that including me.