A variant of "don't bite the hand that feeds you", 'Don't shit in your own backyard' means don't trash a good thing, take advantage of or ruin a close relationship. If you have a positive situation or loving or giving person in your life, you should be careful to protect it/them. When you abuse the situation, person or relationship and deliberately commit an act that you know is potentially harmful or injurious to the situation or person that is good to you, you run the risk of ruining it. Any act that you know would hurt the one with whom you are affiliated and potentially ruin the relationship forever if they found out. The hospitable one (the house with the yard or security) is fouled or injured by your actions (shit upon) which ultimately poisons the soil (the growth medium, sustaining qualities that nurture). Conscious or willful sabotage through selfish actions that generally involves planning and premeditation but can be impulsive. An ultimate act of betrayal through a degree of separation that taints or destroys a perfectly good thing or relationship for no good reason.
Examples of "don't shit in your own backyard" would include pursuing/sleeping with your best friend's or bosses' spouse or significant other; pursuing/sleeping with a mentor's friend; pursuing/sleeping with a friend's close friend that you're interested in; using a person to get close to another; stealing your parent's/friend's car; ditching a loving parent for another adult; keeping secrets you know would further hurt the injured party; wiping the house you're staying at clean; stealing from a friend or relative.
by O'Katalina January 15, 2014