Q: What does the phrase "what side your bread is buttered on" really mean?
A: Thanks for your question! According to the American Heritage Dictionary of Idioms, to "know which side of one's bread is buttered" means to be "aware of where one's best interests lie, as in Jerry always helps out his boss; he knows which side of his bread is buttered. This expression alludes to the more favorable, or buttered, side of bread and has been used metaphorically since the early 1500s."*
*CredoReference, American Heritage Dictionary of Idioms (2003).
(1) Todd, Mark. Take Care of Billy #28. 7 Apr 2008 <http://www.GIANTROBOT.com>