When an owner of a dog takes it for a walk, bags up the dog shit and throws it into a tree where it proceeds to dangle from the branches, therefore producing shit trees. Usually found in rough estates around the UK.
The Shit Tree is a most unfortunate place to sit. Don't take a picnic there whatever you do. One who is having a particularly crappy day would say - I have been sitting under the Shit Tree today. Like Newton noted the apple falling, so would you note the Shit falling. Raining down on your head.
Poor Dave - he's been sitting under the Shit Tree all day.
Charlie's so unlucky - she might as well have been born under the Shit Tree.