Know from a bar of soap
Usually expressed in the negative e.g. “I don't know him/her from a bar of soap” and is thought to be of Australian origin from the early 20th century. It is a variation of “I wouldn’t know him from Adam”, which is a lot older, from at least the 18th century. There are many such sayings of similar construction, too numerous to mention, where virtually anything or anyone can be substituted for Adam or a bar of soap, for any or no reason. Why 'a bar of soap' is anyone’s guess because no one knows for sure.