Siridar is a Galach term for "planetary".
It is possible that the Galach siridar, Fremen al-asiridar derive from the original Arabic as-sadar i.e. "voice" [of the Emperor], this usage presumably referring to the fact that the designated official speaks in the Emperor's name to stipulate what will happen. It could also come from Persian Sirdar/Sardar, signifying a military or political leader, and literally meaning "head holder".
Perhaps, derive from English sidereal (astral, stellar: of the star), which come form Latin "sidus" (star), referring who rule a star system.