a dead cat bounce
a dead cat bouncea misleading sign of vitality in something that is really moribund. informal
A dead cat might bounce if it is dropped from a great height: the fact of it bouncing does not reliably indicate that the cat is alive after all. The expression was coined in the late 20th century by Wall Street traders to refer to a situation in which a stock or company on a long-term, irrevocable downward trend suddenly shows a small temporary improvement.