The word "迫不及待" is formed by combining two vivid ideas to create a powerful image of extreme eagerness. It literally presents the feeling of being "so urgent" (迫切) that one "cannot even wait for a moment" (不及待). This isn't a dry description but a compressed, dramatic scene of a person's inner state, where their desire or need is so intense it visually overflows the bounds of ordinary patience, making any delay feel physically unbearable.