Not attractive, not impressive, nothing about it that catches your eye. It describes first impressions — how a person or a place strikes you before you know anything about it. An unprepossessing man in a grey coat; an unprepossessing little restaurant down a side street. The key thing is that it is gentle. It does not mean ugly and it does not mean bad; it means plain, forgettable, the sort of thing your eye slides over. That gentleness is why it appears so often just before a "but": the unprepossessing restaurant serves the best noodles in town, the unprepossessing man turns out to be the company's owner. Writers use it to set up a surprise. Behind it is prepossess, an old verb meaning to take hold of someone's good opinion in advance — so a prepossessing person wins you over on sight, and an unprepossessing one simply does not. That original verb has almost died out; today the word appears far more often with the un- than without it, which is unusual and worth noticing. This is a literary, formal word. You find it in novels, book reviews and travel writing, and it would sound strange in ordinary conversation — a friend would say "nothing special to look at".

其貌不揚的、貌不驚人的。台灣講「其貌不揚」多半用在人身上，但英文 unprepossessing 也常形容店面、房子、外觀。注意它不是「醜」，比較接近「不起眼」，而且常常是為了後面接一句「但是……」。