There are ghosts, iron, life and Luqi. Never heard of "big watermelon". For example, "I believe you are a ghost" (you are full of nonsense), "I believe you are a big head ghost" (you are full of nonsense, but you don't believe it), "I believe you are iron" (I believe you are a hammer), "I believe you are a fate" (I don't believe you when you die) and "I believe you are an honest man" (I believe you are a root XX). The origin is that those people used to talk about ghosts when burning paper, but now they are used to express their distrust and strong condemnation of each other.