Senatus Populus Quiritium Romanus

<>S most assuredly stood for Senatus - "Senate".
<>P is disputed, some see in it Populus or Populusque, "the people" and "and the people", respectively.
<>Q que ("and"), or Quirites or Quiritium (both of which mean "Spearmen". Originally all Roman citizens had been soldiers.)
<>R probably stood for Romae, Romanus or Romanorum, translated into "of Rome", "Roman" or "of the Romans", respectively.
S.P.Q.R. is an initialism in Latin that was emblazoned on the standards of the Roman legions and was used by the Roman Republic and the Roman Empire