Something that's inadvisable is a pretty bad idea. It would be inadvisable to drive around town with your friend riding on the hood of your car.
Things that are unwise or foolish can be described as inadvisable. It's inadvisable to dress your cat in doll clothes, or to jump off the roof of your shed into a pile of leaves. In both cases, the possibility of getting hurt outweighs the entertainment value. If you wouldn't advise a friend to do something, it's inadvisable.