Same word as unprepossessing:

animal, appalling, awful, bad-looking, beastly, deformed, disfigured, foul, frightful, grisly, gross, grotesque, hard-featured, hideous, homely, horrid, ill-favored, loathsome, misshapen, monstrous, not much to look at, plain, repelling, repugnant, repulsive, revolting, unbeautiful, uncomely, uninviting, unlovely, unprepossessing, unseemly, unsightly