Anything unconsidered is overly hasty or rash — it's not well thought out. You may come to regret your unconsidered promise to buy ice cream for everyone in your grade.
You can also use the adjective unconsidered to describe things that are unappreciated or ignored. If your favorite book has been unconsidered by readers and critics, you should write some positive reviews and post them online. The more common meaning is "careless or reckless," like your unconsidered leap off the roof of the garage into a pile of leaves. Maybe you should have thought that through first.