English to English

adverb (r)

• in a thoughtless manner(adv.all)

  Example:
  He stared thoughtlessly at the picture.
  

  Synonym:
  unthinking, unthinkingly
  

  source: wordnet30
• showing thoughtlessness(adv.all)

  Example:
  He treated his parents thoughtlessly.
  

  Antonym:
  thoughtfully
  

  source: wordnet30