1640s, "pertaining to Eleusis," town outside Athens, site of the mystery associated with the cult of Demeter, goddess of harvests, and her daughter.
The name is literally "arrival" (eleusis), from eleusomai "to go, come," from PIE *elu-to-, from root *leudh- "to grow up, come out" (see liberal (adj.)). This is also the root of Greek eleutheros "freedom."
The best agreement (semantically and formally) to this old ablauting verb is found in Celtic, with the OIr. preterite lod, luid "I, he went" ... formally as good, but semantically less convincing, is the further comparison with Skt. ró(d)hati, Go. liudan "to grow, rise" (whence the old word for "people", OHG liut, etc. ...). [Beekes]