Know which side your bread is buttered??

Could somebody please settle this dispute between myself and my wife regarding this old saying?

Any contributions to the origin/meaning/etc

thanks

To know which side your bread is buttered on means to know what's advantageous for you and what isn't. It means you know how to select courses of action to pursue, people to cultivate, and so forth that are in your interest and might be useful to you. (Metaphorically, you choose the side with the butter.)