Ϻ

Ϻ U+03FA, Ϻ GREEK CAPITAL LETTER SAN ← Ϲ [U+03F9] Greek and Coptic ϻ → [U+03FB]

English Wikipedia has an article on:

san (letter)

Wikipedia

This letter is derived from the Phoenician letter 𐤑 (ṣ, “tsade”)

Ϻ • (Ś) (upper case, lower case ϻ)

1. (historical) The letter san, which was used for the sound /s/ but eventually abandoned in favor of Σ.