A greenstick fracture is a bone fracture seen almost exclusively in children. The term greenstick is derived from an analogy between the pliable bones of children and the flexible nature of a young twig.
Think about when you break a young stick (green ok) and it won't snap it just threads at the point where you have twisted it to get the stick to break. It's this threading that has been assimilied to this particular kind of fracture. Hope you're not suffering.
When I worked in a school, I remember a mum telling me that 'little Johnny' hadn't been at school, as he 'had fractured his greenstick'. It took great willpower to keep my face straight.