A decision that's not very smart or well thought out can be called injudicious. It would be injudicious to spend your last five dollars on a fancy coffee drink.
When you regret something you've done, you might decide in retrospect that it was injudicious. It's injudicious to spread rumors about a friend, because it's not thoughtful. It's also injudicious to ride in a fast-moving car without a seat belt, because it's dangerous. Judicious means "showing good judgment," from the Latin root iudicium, or "judgment."