i don't know how to respond to that
When you're so clueless or too scared to say anything, you just throw this at someone like a lazy kid who refuses to do homework. It’s like the conversation just died from embarrassment.
'I don't know how to respond to that', said by a guy who just got caught eating a muffin at 3 PM on a Friday.
'I don't know how to respond to that', said by your mom when you told her you failed math again.
'I don't know how to respond to that', said by your crush after you accidentally texted them a photo of your lunch.