fem. proper name, from Greek Kasandra, Kassandra, daughter of Priam of Troy, seduced by Apollo who gave her the gift of prophecy, but when she betrayed him he amended it so that, though she spoke truth, none would believe her. Used figuratively since 1660s.
The name is of uncertain origin, though the second element looks like a fem. form of Greek andros "of man, male human being." Watkins suggests PIE *(s)kand- "to shine" as source of second element, hence possibly "praise of men."