dream that goes against reality; dream that predicts the opposite
Refers to a dream whose content is contradicted by actual events, often used in the context of dream interpretation where a dream is believed to foretell the opposite of what will happen.
I dreamed I failed the exam, but it seems it was a dream that predicted the opposite, and I passed.
I dreamed I won the lottery, but it was a dream that went against reality and I didn't win.
Compound of 逆 (saka, 'reverse, opposite') and 夢 (yume, 'dream'). The exact historical origin is uncertain, but it reflects the folk belief that dreams can be interpreted as omens of the opposite outcome.