This expression is extremely hard to explain. It's pronounced long with a hanging intonation, to express disapproval. In this usage, it's often accompanied by a knowing smirk and head-tilt to express the speaker's disappointment, surprise or even disbelief. The hanging intonation purposely makes you think they're going to say more, but the whole idea is that they actually don't have to because they feel that whatever they say, well, goes without saying. In other words, they don't need to say more because the problem is glaringly obvious.
Nicky: [looking at his watch and shaking his head in mock disbelief] Ya know...
Bruce: I know! I know! I just want to finish posting all the photos from the Adams wedding so they can start ordering prints.
Nicky: You'd think after the week you had, you'd want to go home early...
Bruce: I'll tell you what... Tomorrow's Friday. I'll come in late, leave early, and take a long weekend. How's that?
Nicky: Like music to my ears... and to your family's ears, too, I'm sure. See you tomorrow.
____________Here, Nicky didn't have to elaborate after ya know... because Bruce already knew what he was thinking.Amy: Hey, how come you weren't at Sarah's baby shower yesterday?
Nina: Well... I wasn't invited.
Amy: What? Don't you live right down the street from her?
Nina: Yep.
Amy: Well, if it's any consolation, she has a tiny apartment. She probably couldn't fit more than the six people she invited, so she probably just invited her closest friends.
Nina: Yeah, well, I seem to recall five years ago when she had her nose done and all those "closest friends" left town for the weekend, and I took care of her for three days straight even though I barely knew her.
Amy: Seriously? I never knew that.
Nina: Yep. I happened to be jogging past her place when a friend brought her home from the hospital. She looked terrible — she could barely walk. I asked if anyone was going take care of her and she was like "no." So out of the goodness or my heart, I went by like six times a day, gave her her medication — even bathed and fed her. So, ok, you have a small apartment, but ya know...