含义：粗略的、快速估算的（通常指未经详细计算的草稿）。
例句：It was just a back-of-the-envelope calculation, not meant to be precise.
含义：非常偏远、荒凉的地方；天涯海角。
例句：They live in the back of beyond—there’s no cell service for miles!
含义：潜意识里；隐隐约约记得或担心某事。
例句：I had a nagging worry at the back of my mind all day.
含义：对某事非常熟悉（常用于“know like the back of your hand”）。
例句：She knows this city like the back of her hand.