There are several synonyms that can be used to describe the idea of "most secretiveness". These include confidentiality, hiddenness, privacy, secrecy, hush-hush and reticence. When someone is being highly secretive, they are keeping information to themselves and not sharing it with others. This could be for various reasons including the desire to protect themselves or others, to maintain a sense of control, or to retain power or influence. Whatever the reason for their secretiveness, these synonyms can help to convey the level of discretion and privacy they are displaying, and the extent to which they are willing to keep certain things hidden from public view.