comme un dératé

Literally, "like a spleen-less person". The original form was courir comme un dératé. The Ancient Greeks attributed stitch (a stabbing pain in the ribcage due to exercise) to the spleen, and although the actual ablation was never successful, the idea that a person without a spleen would run faster was prevalent in the 18th and stuck in this idiom, which was eventually extended to any sort of quick movement or effort.

• IPA^(key): /kɔm œ̃ de.ʁa.te/

comme un dératé

1. (simile) Very fast.
2. (simile) Very hard and for extended periods of time.