Syl was an older Alzhedo term for a tribal chief that later was used as a prefix. Pasha was a title for a guildmaster of the merchant class, so together, the term "syl-pasha" most literally meant "chief guildmaster".
How this came to refer to the ruler of Calimshan was due to a complicated idiosyncrasy of how one referred to another's titles in Calishite society. An upper-class Calishite person would have many titles for each area of authority he or she possessed. For example, a ruler of Calimshan might have the titles: Syl-Pasha of All Pashas, Syl-Sabbalad of Sabbalads, Syl-Ralbahr of the Navy, Syl-Qayadin of the Armies, Syl-Ynamalik of the Lands of the Three Rivers, Syl-Sultan of the Cities, and Caleph of Our People. It was custom that the lowest title be always listed first. Because it was time consuming, one was permitted to omit the later titles by use of a special hand gesture called an annuv. Doing such would shorten the ruler of Calimshan's title to "syl-pasha".