Plop! was a humor magazine published by DC from 1973 to 1976. I bought most of the copies when they were on the newsstand, and I still have them. It was an unabashed rip-off of Harvey Kurtzman's 1950s MAD, and not as good. The best thing about this self-described "New Magazine of Weird Humor!" were the covers by Basil Wolverton (who drew for MAD) and the marginalia by Sergio Aragones (another MAD artist).
The Bristol Board posted a high-res scan of this unpublished original Plop! cover by Wolverton/Aragones. What does it mean? I don't think it means anything, which is classic Wolverton.
See also: Creeping Death from Neptune, a treasury of early Wolverton comics