The phrase "someone couldn't care less" is often used to describe a person who has absolutely no interest or concern about a particular situation or person. There are several synonyms that can be used to replace this phrase and convey the same meaning. Some of these synonyms include "apathetic," "indifferent," "unconcerned," "nonchalant," "detached," "disinterested," "uninterested," and "lackadaisical." These words can be used interchangeably with "couldn't care less" to express different degrees and aspects of apathy or indifference. Whatever the situation, these synonyms can help to add variety and depth to writing or conversation.