An odd term on the face of it for attractive (although one hears it far more often now in its negative form, unprepossessing.) Off to the OED where I find that prepossessing initially meant ‘that causes bias or prejudice’ (and what a wonderful book title, the source of the first cite)
a1635 R. Sibbes Bowels Opened (1639) 495 Empty the soule of all sinne and prepossessing thoughts.
From thence the meaning narrowed to mean causing a favorable bias, ie attractive. (Much the same process as with success, which originally meant simply outcome, whether good or bad.)
2. That predisposes favourably; giving a favourable first impression; attractive, pleasing.
1737 J. Hewitt Tutor for Beaus 3 That free, open, affecting, prepossessing , graceful, and excellent Air.
And thinking about ugly it’s instructive that the root is the Old Norse word ugglig, to be feared or dreaded.
BTW do Americans use the simile like the back-end of a bus to mean ugly? (Although I’ve never been sure as to what is so fugly about the back of a bus. Of course it depends on the year in which the phrase was coined and which model of bus they had in mind. I can’t find it in OED so I have no idea of date although it was certainly around in the 60s.)
And I’d always thought fugly an American term but no, it’s of Australian military origin and first used as a noun, in other words a fugly was a ‘fucking ugly’ woman. First cite is as a noun c. 1970 and as an adjective in 1980.