bless your heart
A group of protesters demonstrating against President Trump's then-forthcoming speech in Nashville, Tennessee, on Wednesday dealt with a pro-Trump counter-protester in their midst with classic Southern hospitality. As the man wearing a "Make America Great Again" cap began shouting into the crowd, they responded with a hearty chant of "Bless your heart!"
For those unfamiliar with the subtle and varied nuances of "bless your heart," it is a "tremendously flexible phrase" whose meaning can range from sincere sympathy to cutting dismissal. In this case, its usage as "a passive-aggressive way to call you an idiot" is probably about on the mark.