习语 a bad penny always turns up (again) 是什么意思？

英语习语 a bad penny always turns up (again) 是一个谚语，主要用来表示不受欢迎（或讨厌）的人总是一再出现（an unwanted or unsavory character is likely to show up when least expected）。比如：

• "Who's coming?" Father asked. "It's Uncle Joe, the drunkard," I answered."Oh, that's the case of a bad penny always turning up again," Father said with a sigh.
  “谁要来呀？”父亲问。“是酒鬼乔叔，”我回答说。“唉，这真是不受欢迎的人偏要来纠缠，”父亲叹了一口气说。