Why is the name Cassandra synonomous with a person who is a fortune teller or other psychic phenomenon? - Jinx
Because Cassandra was the name of a girl from the Iliad she was given the gift of prophecy by Apollo but spurned him, so he gave it a twist. She was doomed to tell the truth, but never be believed.
Aha! So today it’s used for irony: to speak lies but always be believed! Clever.
See here for more detail of the legends.
Thanks Silenus, I was going to link to Wikipedia but it’s down right now.