The story
According to OED, from a genus name Isidium (Acharius 1798), itself from the goddess Isis.
Etymology from Wiktionary, CC BY-SA 4.0.
isidium
Today it means: A vegetative reproductive structure present in some lichens; an outgrowth of the thallus surface, usually with a columnar structure, and consisting of both fungal hyphae and algal cells. Full definition →