I don't know what to say

Often used when someone says something that is obviously a wrong/mistake/undependable, and the person who is listening can't say anything to defend his actions.

Jack: I missed the deadline for applying to NYU, because I read the email wrong.

Sam: They gave us a one week extension for that exact reason? Why didn't you submit it.

Jack: I was just so lazy about it.

Sam: I don't know what to say Jack, I guess that's a lesson learned.