An ill-conceived idea is one that hasn’t been carefully considered. Your ill-conceived plan to ride your skateboard from New York to California is bound to turn out badly.
When you don’t spend enough time planning or thinking things through, the result is likely to be an ill-conceived scheme of some kind. Ill-conceived laws tend to have unfortunate effects that lawmakers didn’t expect, and ill-conceived movie scripts result in films whose plots make no logical sense. Something that’s conceived is thought of, planned, or imagined, and when you add the prefix ill, you get something that’s planned badly.