Other forms: unwisest; unwiser
Doing something unwise means acting in a foolish or irrational way. It would be unwise to bring your cat to the library with you — you'll probably get in trouble, and the cat is unlikely to enjoy it.
Wise means "having sound judgment or intelligence," and when you add the prefix un-, or "not," you get the exact opposite. Making an unwise choice means you're not using your good sense to consider all possible consequences. It would be unwise to wear shorts on the coldest day of the year, and even more unwise to walk barefoot to school in the snow. Use your common sense and wisdom and you'll avoid making unwise decisions!