Think of the look and feel of a sick room: a thin body that never quite gets strong, a face with no colour in it, and air that is too sweet and too still. Sickly takes that whole room and lends it to other things. The word never says a thing IS ill. It says the thing has the look, the colour, the smell or the weakness of illness — and that you would rather not be near it. That dislike is always part of the word, which is why almost nothing described as sickly is being praised.