unprepossessing

Roddyは "sent to Coventry" になったはずですが、Alainは何故か彼に近づきます。
The scene was so bizarre that when the other gamers stormed off and left Roddy to himself among the corpsed, Alain stayed and started to talk to him ... and after about five minutes of conversation realized that here was one of those people around whom life would never be dull. Granted, Roddy looked dull enough: big but pudgy for seventeen, with a blobby, unfinished look to him, wearing an unprepossessing face in which nothing but his eyes seemed alive.
上の引用文の後半に出てきた "unprepossessing" という単語は、分解すると "un" + "pre" + "possessing" でそれぞれの意味は分かりますが、合成された "unprepossessing" の意味は分かりません。しかたが無いので辞書を見ます。
・Oxford English Dictionary: Not particularly attractive or appealing to the eye: ‘despite his unprepossessing appearance he had an animal magnetism’
・Collins Dictionary: not creating a favourable impression; unattractive: A squat, unprepossessing face, moustache, tiny round glasses that gave an intellectual air.
意味は分かりました。Vocabulary.comを見ると "unprepossessing" のニュアンスが良く分かります。
If you find someone to be unprepossessing, you find them unattractive. Not that they're ugly, mind you! Just unprepossessing.
Unprepossessing is a rather indirect way of calling someone unattractive, or at best OK-looking. Unprepossessing is not quite the same as "ugly." Rather, just a way of saying that someone's looks aren't what you're most likely to remember about them. Cinderella was most unprepossessing in the filthy clothes and worn-out shoes that her step-sisters forced her to wear. But when she was all decked out by her fairy godmother, she was the belle of the ball: she was no longer unprepossessing.
しかし、何故この様な意味になるのかは依然として疑問です。Etymology.comで "prepossess" の語源を見ると。
1610s, "to get possession of beforehand," from pre- + possess. Meaning "to possess (a person) beforehand with a feeling, notion, etc." is from 1630s; specifically, "to cause (someone) to have a favorable opinion of something" (1640s).
これなら "unprepossessing" の意味とつながりますね。