Definition - What does Hornbook mean?
A hornbook is a legal primer book describing fundamental legal principles and defining basic legal terms. When the word "hornbook" is used as an adjective, it denotes an idea, concept or body of law that is considered so fundamental that there is no room for questioning the validity of the idea.
Justipedia explains Hornbook
Before the electronic age, novice legal scholars began their education with a hornbook. Hornbooks taught the basic vocabulary and philosophy that would serve as the baseline knowledge that legal scholars had to master before studying more advanced legal concepts.
If a court describes an idea as hornbook, the court is saying that it does not need to describe the idea any further because all lawyers should already understand that idea. The term can connote a court's frustration with needing to address an issue described as hornbook.