When you're thoughtless about something, you don't spend much time thinking about what the consequences or effects on others might be. Smashing your neighbor's Jack-o-lantern was a thoughtless impulse you immediately regretted.
If you've ever said, "I didn't really think about it hurting someone's feelings—I just did it," you've acted in a thoughtless way. This adjective is always at least a little bit negative, basically the opposite of thoughtful. It's thoughtless to invite everyone except the new kid at school to your birthday party, and it's thoughtless to throw trash on the ground instead of in a trash can. It's always better to stop and think first!