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!

Definitions of thoughtless
  1. adjective
    showing lack of careful thought
    “the debate turned into thoughtless bickering”
    lacking regard for the rights or feelings of others
    inconsiderate, unconsidered
    without proper consideration or reflection
    unreflective, unthinking, unthoughtful
    not exhibiting or characterized by careful thought
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    exhibiting or characterized by careful thought
    showing concern for the rights and feelings of others
    bemused, deep in thought, lost, preoccupied
    deeply absorbed in thought
    brooding, broody, contemplative, meditative, musing, pensive, pondering, reflective, ruminative
    deeply or seriously thoughtful
    given to cogitation
    well thought out
    resulting from careful thought
    involved in or characterized by deliberation and discussion and examination
    concerned with excogitating or having the power of excogitation
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  2. adjective
    without care or thought for others
    “the thoughtless saying of a great princess on being informed that the people had no bread”
    synonyms: uncaring, unthinking
    lacking regard for the rights or feelings of others
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