a plastic bag, or more pertinently, a PVC or similar jacket proper waterproof. The snag is, if you wear it whilst doing any exercise, you'll get soaking wet from sweat, so waterproofness is hardly relevant. For hillwalking, I wear Paramo stuff, which is kind-of waterproof, but very breathable and you don't get wet inside. Sadly they don't make a cycling jacket, and it would probably be a bit warm. For cycling I wear a windproof fleece, very slightly water-resistant thing, which keeps me warm, and doesn't get wet inside so long as I'm active.
So the simple "is it waterproof" question, is really the wrong question, silly though that may sound