She is so heinous that you would rather chop off her head and replace it with a Jack-o-lantern, take care of your business and then move on with your life. Hence the word 'Jackie.'
Jackie's are most commonly girls that you only see from behind and first and are misled to think is hot.
Adam: Which one?
Sean: The one over there with her back to us.
(She turns around)
Adam: Aw! She's a Jackie.