Good fucking question.
Back in 2011-2012, "social justice warrior" was a phrase used on Tumblr by social justice bloggers, to distinguish themselves from people who used violent rhetoric in discussions of equality. Tumblr user "crackerhell" is probably the best example, as he openly told white people to kill themselves for appropriating other cultures (which included wearing a kimono) and claimed to be happy that "27 white kids died" after Sandy Hook. These people were very real, but there were only a few thousand of them. They spent much of their time arguing with less violent social justice bloggers who generally had the same end-goal: equality and an end to oppression.
Around 2013, the term became increasingly proliferated across the internet, mostly through reddit. Pretty quickly, the term became synonymous with all radically progressive bloggers, even those who didn't advocate for violence or bully others. /r/tumblrinaction can be hilarious, but pretty often it involves mocking people for their appearance or gender identity, two things that aren't hurting anyone and honestly should not be fair-game for criticism. SJWs became associated with a preference for non-binary pronouns, and then for any serious activism for change.
These days, after the shit-flinging that is Gamergate, an "SJW" is really any kind of advocate for social justice. All feminists in the media will be described with that acronym, even incredibly harmless people like Tina Fey. Hell, edgy racists on 4chan would insist that Abraham Lincoln was an SJW. The term has really lost its meaning, due to major overuse by more conservative parts of the internet.