i don't know how to respond to that
This is when you're either too confused or too lazy to say anything, so you just throw it out there like you're trying to save the conversation from certain death.
'I don't know how to respond to that', said by your sister when you told her you're dating her ex.
'I don't know how to respond to that', said by your coworker when you told them you're getting a promotion.
'I don't know how to respond to that', said by your crush when you told them you're moving to another country.