This is a term that your siblings use to bully you. It describes someone who is fat and flabby whose man boobs flap together when they run... hence the term flappy. It has the potential to cause resentment and damage your childhood. But it’s okay because one day, when you and your siblings grow up, you will be better looking, more successful, and happier than them!
Sibling: Flappy, do you know who the milkman is?
You: No, who is he?
Sibling: Your father!