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 this at them like you're trying to escape a burning building.
'I don't know how to respond to that', said by your dad when you told him you're moving out.
'I don't know how to respond to that', said by your friend when you told them you're dating their best friend.
'I don't know how to respond to that', said by your crush when you said you're going to college 3000 miles away.