Samekh or Simketh is the fifteenth letter in many Semitic alphabets, including Phoenician, Hebrew, and Aramaic, representing /s/.
The Phoenician letter is where the GreekXi (Ξ, ξ) comes from.[1]
↑Muss-Arnolt, W. (1892). On Semitic Words in Greek and Latin. Transactions of the American Philological Association v. 23, p. 35-156. The Johns Hopkins University Press.