Fuck it, let's try an Academic approach.
Social Justice.
Line breaks: so¦cial jus|tice
Noun (mass noun)
Justice in terms of the distribution of wealth, opportunities, and privileges within a society:
individuality gives way to the struggle for social justice
Source: http://www.oxforddictionaries.com/definition/english/social-justice?q=social+justice
Now, I can't find a solid dictionary definition for Social Justice Warrior, but I did find several sources which gave an attempt at a definition. Here is a relatively unbiased one:
Social Justice Warrior
Noun
A Social Justice Warrior or SJW is someone who takes to heart various causes that pertain to Social Justice. Be it fighting for People of Colour (PoC) or Gender/Sexual Minorities (GSM), the Social Justice Warrior usually believes in intersectionality, the idea that oppression occurs in society across a variety of mediums and is all controlled by a central systematic culture, or Kyriarchy. SJW are most prominent on their safe space, the internet, normally using either tumblr or live journal to blog on.
Source: http://internetobservationproject.com/social-justice-warrior-definition/
Between the two above, I have provided what can be seen as the basis of a Social Justice Warrior. However, you want the meaning of slang, every day term. I have some more sources, but these will seem biased, but all semantic meaning is. I have chosen a very popular source which would have had the most public scrutiny; Urban Dictionary, where it is the public who submits the definitions and effectively edits it in "trial by fire".
Social Justice Warrior
A pejorative term for an individual who repeatedly and vehemently engages in arguments on social justice on the Internet, often in a shallow or not well-thought-out way, for the purpose of raising their own personal reputation. A social justice warrior, or SJW, does not necessarily strongly believe all that they say, or even care about the groups they are fighting on behalf of. They typically repeat points from whoever is the most popular blogger or commenter of the moment, hoping that they will "get SJ points" and become popular in return. They are very sure to adopt stances that are "correct" in their social circle.
The SJW's favorite activity of all is to dogpile. Their favorite websites to frequent are Livejournal and Tumblr. They do not have relevant favorite real-world places, because SJWs are primarily civil rights activists only online.
Source: http://www.urbandictionary.com/define.php?term=social%20justice%20warrior
Now, Know Your Meme does cover the criticisms of the Social Justice blogging movement.
However, the group has been criticized for propagating unreliable information and espousing slacktivism and herd mentality, as reflected in the pejorative term ?social justice warriors.
...
Criticism:
The concept of social justice blogging been criticized by outsiders of the community as being overzealous and self-righteous. In response to the pervasive nature of social justice bloggers on Tumblr, anti-social justice blogs had begun to surface by early 2011, including Fuck No Tumblr Social Justice Warrior[8] in May 2011, Shit Social Justice Allies Say in May 2012, Silly Social Justice in June 2012 and Fuck No Social Justice Warriors[9] in July 2012.
To summarise an actual answer to your question, I shall try to provide a definition to Social Justice Warrior.
Social Justice Warrior.
(see: Social Justice, Social Justice Blogging, "Check Your Privilege")
Noun
1. A person who takes it upon themselves to actively campaign for issues of social justice, meaning the imbalances of sex, race, gender, wealth and other possible privileges.
2. (Pejorative in nature) A person who is seen as overzealous, unreasonable, uneducated or hypocritical in their views or statements in regards to support of social justice
Source: I thought of it, so I'll write it up later and slap on a source. See below.
https://leon-declis.squarespace.com/articles-on-society/sjw-definition BAM!