be unprepossessing in appearance貌不惊人
ADJ(形容词) If you describe someone or something as unprepossessing, you mean that they look rather plain or ordinary, although they may have good or special qualities that are hidden. 其貌不扬的 [正式]
We found the tastiest tapas in the most unprepossessing bars.
我们在最其貌不扬的酒吧找到了最好吃的西班牙餐前小吃。