The phrase "cast slur on" refers to the act of making a damaging, false statement about someone or something. The opposite of casting a slur would be to speak positively, honestly, and truthfully. Thus, the antonyms for "cast slur on" could include praising, complimenting, lauding, extolling or eulogizing someone or something. These words convey respect, admiration, and appreciation for someone or something, rather than attacking or insulting them. Instead of casting a slur on others, it is always better to use kind and positive words to celebrate their greatness, to build them up and to create an atmosphere of honor and respect.