i don't know how to respond to that
This is when you’re either too dumb or too scared to say anything, so you just throw it out there like a weak excuse. It’s like the conversation just gave up on you.
'I don't know how to respond to that', said by your friend after you told them you slept with your dog.
'I don't know how to respond to that', said by your teacher when you showed up in pajamas on a Monday.
'I don't know how to respond to that', said by your crush after you said you don't like them anymore.