"Senatus Populusque Romanus" means "the Senate and People of Rome."
Obelix and Asterix say, "sono pazzi, questi romani!" ("They're crazy, these Romans!")
Read more about it all at http://en.wikipedia.org/wiki/SPQR.
NOTE: Wikipedia, at the time of this writing, says "(The latter were originally placed by order of Mussolini, who frequently used SPQR as propaganda for his regime.)" I have seen, however, manhole covers with dates from before the Mussolini epoch but well into the modern era, like the late 1800s and early 1900s. Sure, the guy pushed it, but I don't think they were "originally placed" by him.
On to the PROOFSHEET! 527 examples of SPQR, last updated October 2006.
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