For the uninitiated, the term is somewhat au courant these days and refers to people (okay, almost always female) who are "not usually in the conversation of being among the hottest of their group, but when mentioned or seen you remember that they are rather attractive."
It could, in my view, have a temporal component as well, describing someone you've seen or perhaps known for a while, who didn't initially strike you as being all that, but one day you look at them and see a certain attractiveness that you didn't notice before. Not really the "homely librarian" type of thing where a new outfit, hair style, and ditching the thick glasses transforms the ugly duckling into a graceful swan; we've all seen that schtick before, it's been the plot line of hundreds, probably thousands, of movies, TV episodes, and romance novels.
Not like the cinematic treatments are all that believeable, of course; you can't just slap a pair of thick glasses on a smokin' hot actress and make her appear plain to anyone blessed with the gift of sight. No, this is more a function of a different, shall we say, perspective of the viewer rather than a different look for the viewee; and it can often take a long time for that perspective to change.
In the case of me and the VW Rabbit, it has taken most of my adult life. But I finally got there.
I honestly never gave the Rabbit/Golf a second look, except perhaps as this odd-looking "hatchback" thingie that I would never, ever Lust over, let alone want to drive and/or own. Even the famed GTI didn't blow my youthful skirt up, weaned as I was on ground-pounding American muscle cars as my performance icons. Nah, that goofy little square-looking front-drive Beetle-wanna-be wasn't my cup of tea and never, I assumed, would be. Oddly enough, however, it wasn't even a Mk1 that first piqued my interest: it was a plain, white four-door late model Golf that used to drive by every morning while I was waiting for the bus. For whatever reason, it began to strike me as a subtly sexy little thing; not supermodel hot, but more, well, sneaky hot. And I began to think "Hey, maybe those old Rabbits weren't too shabby after all. . . ."