The signature of modern leftist rhetoric is the deployment of terminology that simply cannot fail to command assent. As Orwell himself recognized, even slavery could be sold if labeled "freedom." In this vein, who could ever conscientiously oppose the pursuit of "social justice," -- i.e., a just society?
To understand "social justice," we must contrast it with the earlier view of justice against which it was conceived -- one that arose as a revolt against political absolutism. With a government (e.g., a monarchy) that is granted absolute power, it is impossible to speak of any injustice on its part. If it can do anything, it can't do anything "wrong." Justice as a political/legal term can begin only when limitations are placed upon the sovereign, i.e., when men define what is unjust for government to do. The historical realization traces from the Roman senate to Magna Carta to the U.S. Constitution to the 19th century. It was now a matter of "justice" that government not arrest citizens arbitrarily, sanction their bondage by others, persecute them for their religion or speech, seize their property, or prevent their travel.
This culmination of centuries of ideas and struggles became known as liberalism. And it was precisely in opposition to this liberalism -- not feudalism or theocracy or the ancien régime, much less 20th century fascism -- that Karl Marx formed and detailed the popular concept of "social justice," (which has become a kind of "new and improved" substitute for a storeful of other terms -- Marxism, socialism, collectivism -- that, in the wake of Communism's history and collapse, are now unsellable).
"The history of all existing society," he and Engels declared, "is the history of class struggles. Freeman and slave, patrician and plebian, lord and serf ... oppressor and oppressed, stood in sharp opposition to each other." They were quite right to note the political castes and resulting clashes of the pre-liberal era. The expositors of liberalism (Spencer, Maine) saw their ethic, by establishing the political equality of all (e.g., the abolition of slavery, serfdom, and inequality of rights), as moving mankind from a "society of status" to a "society of contract." Alas, Marx the Prophet could not accept that the classless millenium had arrived before he did. Thus, he revealed to a benighted humanity that liberalism was in fact merely another stage of History's class struggle -- "capitalism" -- with its own combatants: the "proletariat" and the "bourgeoisie." The former were manual laborers, the latter professionals and business owners. Marx's "classes" were not political castes but occupations.
Today the terms have broadened to mean essentially income brackets. If Smith can make a nice living from his writing, he's a bourgeois; if Jones is reciting poetry for coins in a subway terminal, he's a proletarian. But the freedoms of speech and enterprise that they share equally are "nothing but lies and falsehoods so long as" their differences in affluence and influence persist (Luxemburg). The unbroken line from The Communist Manifesto to its contemporary adherents is that economic inequality is the monstrous injustice of the capitalist system, which must be replaced by an ideal of "social justice" -- a "classless" society created by the elimination of all differences in wealth and "power."
Social Justice: Code for Communism