When Sarah Palin used the term “blood libel” to describe purported attacks on her and the Tea Party movement in the wake of Saturday’s tragic shooting in Tucson, some were left wondering why the former governor would use a phrase historically associated with anti-Semitism.

But, whatever the merits or demerits of Palin’s usage, it didn’t come out of nowhere. And that alone is a useful reminder that the Internet’s a big, diverse place, stocked with ecosystems, subcultures, and communities that each bring their own assumptions about language. For journalists (or anyone), it can be easy to think that your little corner of the Internet is representative of the big picture. It’s probably not.

The use of “blood libel” may seem inexplicable — that is, until you go back and look at how the word was used in particular digital media circles during the days since the Tucson shooting. The Lab has written previously about Internet memes, how ideas tend to move more like heartbeats than viruses through the web’s extremities. And the path “blood libel” took — while, on the one hand, it suggests the social divisions that can live online — also offers some insight into the trip memes take as they bubble up into the consciousness of the mass media.

With a bit of Google News sleuthing, supplemented by a trip to the Lexis-Nexis archive, it appears that the term “blood libel,” pre-Palin, was adopted by some conservative commentators in the immediate aftermath of the Tucson assassination attempt.

The first use of the phrase I uncovered came on January 9, one day after the shooting, on the website Renew America. As conservative activist Adam Graham put it: “When someone on the left says that the Tea Party movement is responsible for the shooting in Tucson, they are leveling the political equivalent of a blood libel that blames an entire political movement for the actions of a person who in all likelihood had no connection to the movement.” Note that Graham links to the Wikipedia page on “blood libel,” demonstrating knowledge of the traditional meaning of the term.

The term really sprang into use, however, when conservative blogger Glenn Reynolds (Instapundit) used it in an op-ed in The Wall Street Journal on Jan. 10. Headlined online as “The Arizona Tragedy and the Politics of Blood Libel,” the piece asked: “So as the usual talking heads begin their ‘have you no decency?’ routine aimed at talk radio and Republican politicians, perhaps we should turn the question around. Where is the decency in blood libel?”

In the days after that piece appeared, more conservative bloggers picked up “blood libel” — and it was further amplified when commentators of a variety of political stripes used the phrase in their discussion of the Reynolds op-ed. Both Reason Online and Associated Content quoted Reynolds’ use of  the term “blood libel” in their broader discussion of political rhetoric and violence. I also found “blood libel” used, without reference to Reynolds, in Human Events, The Washington Examiner, and Big Journalism (in the comment section).

The surest sign that the “blood libel” meme had caught on, though, came when it started to be used in major media comment sections like those of the Washington Post. Ordinary website readers were now referencing the term.

And then came Palin. And here we are.

None of that is a scientific analysis, of course. And as helpful as digital tracking tools like Google and Nexis can be, the fact that “blood libel” was lurking in the web’s shadows in the first place, ready to emerge almost fully formed, suggests the unknowability of the web — its anonymity, its opacity — as much as its readability.

Still, the general path “blood libel” took over the past few days shines some light on how particular terms move within the digital media ecosystem, and how the use of language that seems strange to many — as it did to many commentators, judging on their reactions to it — can appear “normal” to others who are operating within a different discursive community. That’s not to make another lamentation of “cyber-balkanization” or another call for the return of the “mass public sphere” where everyone read and thought the same thing. It is just a reminder, though, that our digital house has many rooms. Sometimes, when you feel like politicians aren’t speaking to you, you’re right. They’re not.