Another answer I came across recently mentioned that orders for specific performance ie mandatory injunctions formerly called mandamus were writs. Conversely so were other writs such as certiorari. What, historically, was a “writ”? Are they a legacy of equity, rather than law, back when they were separate disciplines?
A writ is simply an order issued under legal authority for someone to do or not do something.
Today the word is usually associated with orders by a court but historically, they applied to administrative and parliamentary orders as well. Technically, a “No parking” sign is a writ.
Writs are not unique to equity. Writs developed as essentially template actions and associated orders in both law courts and chancery. They eventually crystalized around a fixed set of wrongs and remedies which developed into modern day categories of action and relief. See generally: Frederic William Maitland & Sir Frederick Pollock, The History of English Law before the Time of Edward I, vol. 1, "CHAPTER VII: The Age of Bracton."