“Reactionary” means regressive conservative/anti-progressive. It originates from, as much political terminology on the regressive vs. progressive divide does (including the terms “left”, “right”, and “conservative” themselves), the French revolution, where people who favored opposing the revolution (i.e. reacting to the revolution) were called “réactionnaires” in French.

Here’s the first known usage of the word in English, from a 1799 English translation of Lazare Carnot’s letter on the Conspiracy of the 18th Fructador.