An insult that doesn't sink in for awhile.
Example : When Jasmine figured out that slow burn comment from the girl at the bar, she was so angry.
Or - He’s an idiot, you can say any sort of stuff to his face and it’ll be a slow burn. He won’t figure it out for days.
My source is the "Urban Dictionary" , but don't go over there most words are too naughty. I clean them up for you. I always clean up after people.