1831, "of or pertaining to political reaction, tending to revert from a more to a less advanced policy," on model of French réactionnaire (19c.), from réaction (see reaction). In Marxist use by 1858 as "tending toward reversing existing tendencies," opposed to revolutionary and used opprobriously in reference to opponents of communism. Non-political use, "of or pertaining to a (chemical, etc.) reaction" (1847) is rare. As a noun, "person considered reactionary," especially in politics, one who seeks to check or undo political action, by 1855.