A shirt overbite occurs when someone is really fat and has a huge stomach. The bottom of their shirt is the same vertical length as the top of their pants but there is a huge horizontal gap between them because their shirt is hanging from their belly. There is a horizontal gap between the end of the shirt and the beginning of the pants, due to their huge protruding belly.
Eli is so fat that when he tries to pick up girls they take one look at his six inch shirt overbite, laugh, and turn around. Eli tries to hide his shirt overbite by wearing long shirts.