Here's an example from an 1897 book review I found in the google books corpus: "There never was anything in all this world quite so provincial as American literature. When we over here are not reading slop we read the books that bear the approval of London." Or this doggerel from the October 1920 issue of Farm Journal: "I read all kinds of printed junk / the good, the noble and the punk / the ancient and the new / and men who see me reading slop / predict that if I do not stop / my head will go askew."
These both seem like wiktionary's current sense 8 ("Content or entertainment which is worthless, or produced and consumed mindlessly"), so I don't see any need to credit 4chan or 21st-century anti-Semites in general.
https://languagelog.ldc.upenn.edu/nll/?p=73708#comment-1642217