An ill-advised decision is one that isn't smart or thoughtful. It would be ill-advised to adopt all the dogs, cats, rabbits, and guinea pigs from your local animal shelter — especially without consulting your family members first.
This adjective is basically a polite way to say "just plain stupid." Political commentators may judiciously criticize a policy decision as ill-advised if they think it's a bad idea. Your mom might respond to your brother's request to let his pet chicken sleep in bed with him by calmly saying, "That would be ill-advised." The word implies that you've been given bad advice, which may be true, but it's generally your own recklessness that leads you to do something ill-advised.