Jack was first called out by a person on Reddit that said he was narcissist (btw in my opinion they are not wrong), and started a trend on his Reddit page.
Narcissistic personality disorder is a mental health condition in which people have an unreasonably high sense of their own importance. They need and seek too much attention and want people to admire them. People with this disorder may lack the ability to understand or care about the feelings of others. (https://www.mayoclinic.org/)
This is the jack motto for narcissism: Jack = Narcissim.