In English law and its descendants, a restraining order is a form of injunction.

After a period of sorting out, the English crown took over the administration of justice in England and Wales. As far as your question is concerned, two separate kinds of courts were of interest: Common Law courts, and Chancery or Equity courts.

The theory was that the Common Law courts would strictly enforce the law according to ancient precepts, or by newer statues if any were relevant. If someone stole your money or occupied your land or killed your cow, you would file exactly the right kind of pleading to start the suit, and eventually the court would have the authority to award some specific kind of relief. The procedures and powers of the Common Law courts were rigidly defined, which had the advantage of making the law stable and transparent, but had obvious disadvantages as well.

Courts of equity had authority over different subjects than the common law courts: matters like wardship, wills, and later trusts and corporations. Chancery courts had quite different procedures and powers compared to common law courts -- including the power to issue injunctions.

It wasn't uncommon for the same dispute to involve problems both of law and of equity. Eventually, in the 19th century, British courts merged the two systems, so that ordinary courts could exercise both common law and equitable powers. The U.S. mostly followed suit later, but now the federal government and almost all states have just one system now. Notably, Delaware, home state of so many corporations, still has a separate Chancery Court.