Not know (someone) from a bar of soap verb phrase idomatic
To have never met someone or to completely unknow anything about someone
Harry is really fond of Victoria but he doesn't know her from a bar of soap.
Last night, a man who I wouldn't know from a bar of soap started a conversation with me.
I wouldn’t know this girl from a bar of soap, but she insisted claiming that she was my distant relative.
To refer to an unfamiliar or faraway place.
Someone who is totally bad at doing something.
Mary really can't sing for toffee.