It's a phrase that sounds good but is rarely examined and debated.
Help is on the way.
There is now a vast database that examines the chief proponents of "social justice" and similar leftist causes.
I found this article that traces the genealogy of the phrase "social justice" back to Marx.
In this rendering, "social justice" is an attack on classical liberalism.
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?This attack on classical liberalism becomes clearer when one recognizes that there is a tension between freedom and equality. Freedom tends to lead to inequality. Equality cannot be achieved without curtailing freedom.
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 regime, 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).
Give Marx his due: He was absolutely correct in identifying the political freedom of liberalism -- the right of each man to do as he wishes with his own resources -- as the origin of income disparity under capitalism. If Smith is now earning a fortune while Jones is still stuck in that subway, it's not because of the "class" into which each was born, to say nothing of royal patronage. They are where they are because of how the common man spends his money. That's why some writers sell books in the millions, some sell them in the thousands, and still others can't even get published. It is the choices of the masses ("the market") that create the inequalities of fortune and fame -- and the only way to correct those "injustices" is to control those choices.