not know one end (of something) from another
To have absolutely no knowledge about something. I don't know one end from another when it comes to computers, so you'll have to explain this to me really slowly. What's an attachment again? Jeff wouldn't know one end of a baby from another. Thank goodness Frank is there to help babysit.
not know/not be able to tell ˌone end of something from the ˈother
(informal) know absolutely nothing about something, for example a machine: Don’t ask me to fix the car. I don’t know one end of an engine from the other.