The phrase "all by one's lonesome" means that someone is alone or isolated. However, there are many synonyms for this phrase that can be used to describe the same thing. Some examples of synonyms for "all by one's lonesome" include "all alone," "solitary," "lonely," "isolated," "secluded," "desolate," "detached," "unaccompanied," and "unaided." These synonyms can be used interchangeably with the original phrase, depending on the context and intended meaning. Using synonyms can add variety and interest to writing, while still conveying the same message.