thoughtless has 2 distinct synonyms spread across 1 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is thoughtful.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
showing lack of careful thought
“the debate turned into thoughtless bickering”
without care or thought for others
“the thoughtless saying of a great princess on being informed that the people had no bread; `Let them eat cake'”
Synonyms, antonyms and the broader and narrower relations for thoughtless come from Princeton University’s WordNet. Full sources and licences.