IPA: /ˈθɔːt.ləs/
KK: /θɔt.ləs/
Not considering the feelings or needs of others; acting without thinking about the consequences.
His thoughtless comments hurt her feelings.
Comparative: more thoughtless
Superlative: most thoughtless
Thoughtless → It is formed from "thought" (from Old English *þoht*, meaning the process of thinking) and "-less" (meaning without). The word "thoughtless" means without thought or consideration.
Think of being 'without' ('-less') any 'thinking' ('thought') — that's why thoughtless means not considering something.