In the Ryukyuan language at the time, the pronunciation was closer to shisshii, and has only changed relatively recently.
The sessei worked alongside the king and the Sanshikan (Council of Three) to draft and enact laws, though the king gradually became more and more of a figurehead over the course of the period when Ryūkyū was a subsidiary of the Japanese feudal domain of Satsuma (1609–1870s).
Like most Ryukyuan government officials at the time, most sessei were appointed from the elite class of yukatchu, scholars of Chinese subjects from the town of Kumemura.
According to the Chūzan Seikan (中山世鑑, "Mirror of Chūzan"), the classical Ryukyuan history text by sessei Shō Shōken, the sessei have always been a part of the system of the Ryukyuan Kingdom and were originally appointed by Eiso.
While most sessei essentially played the role of a bureaucrat and privileged member of the royal entourage, Shō Shōken, who held the post from 1666 to 1673, is particularly known for acting as a lawmaker, issuing a great many important and beneficial reforms during his short tenure.