The phrase "casting a slur on" is commonly used to describe the act of making a damaging or false accusation against someone. The spelling of this phrase is phonetically represented as /kæstɪŋ ə slɜr ɒn/. The "c" in "casting" is pronounced as "k" and the "g" is silent, while "slur" is pronounced as "slɜr". The word "on" is pronounced as "ɒn", with a short "o" sound. The correct spelling of this phrase is important to ensure clear communication and accurate understanding.