Jessa

Jessa (or Jesza in Polish) – Polish or Lechitic god whose exact functions are unknown. Jessa was first mentioned in around 1410 in the postil of Łukasz of Wielki Koźmin in which he was warning about the worship of Jessaa and other gods such as Nija, Lada and Kij during spring rituals and folk performances. According to Jan Długosz, he was the highest of the Lechitic gods and the equivalent to Roman god Jupiter.

I think that Jessa could also be used as a nickname for Jessamine. Jessa seems to me a feminine name that is not as popular as other nicknames for Jessica - such as Jessie, Jessy, or Jess.