There was a previous question asking if it's an anti-concept. I'm having a harder time conceptualizing how it's a stolen concept.
asked Nov 25 '12 at 11:04
The earlier discussion (Feb. 2011) regarding anti-concepts can be found here.
A brief explanation of stolen concepts can be found in the topic of "'Stolen Concept,' Fallacy of" in The Ayn Rand Lexicon.
Technically, "social justice" is a compound concept, not a single concept. The usage of "justice" in that expression certainly appears to be an example of the fallacy of the stolen concept. That expression "steals" the concept of justice. For additional discussion of what "social justice" generally attempts to denote, refer to the earlier discussion linked above, and to the present version of the Wikipedia entry which the earlier Objectivist Answers discussion cites, namely, the entry on "Social justice." Here is a sampling from the current version of the Wikipedia article (Wikipedia articles tend to evolve over time):
Social justice is justice exercised within a society, particularly as it is exercised by and among the various social classes of that society.
I also see the anti-concept aspect of "social justice" as more dominant than the stolen-concept aspect. The expression is aimed at distorting the rational, objective meaning of "justice" and obliterating essential aspects of it.
Update: What Is Stolen?
For further clarification of the "stolen concept fallacy," refer to another, more recent question (link).