There's been much discussion about
Κασσάνδρα (plural
Κασσάνδρες), Cassandra.
According to the
Greek dictionary, it is "a person who constantly makes ominous predictions and estimates, which usually do not come true." This corresponds to the modern use of the word, which disregards Cassandra's original role.
English dictionaries will tell you it is anyone whose prophecies of doom are unheeded, and this is closer to the ancient story, as Cassandra was endowed with the gift of prophecy but fated never to be believed.
I'm not close to my dictionaries now but it'd be interesting to see how the dictionary entry has evolved over the years.