reprendre du poil de la bête

To recover your strength and spirits after a setback

Used for bouncing back after being knocked down, physically or otherwise, and finding your feet again.

Après sa maladie, il a vite repris du poil de la bête.

After his illness, he quickly got back on his feet.

Goes back to an old belief, recorded by Pliny the Elder, that a wound from a dog's bite could be healed by applying a hair from the same dog. The phrase first appears in the 16th century as 'prendre du poil de la bête', and the meaning drifted over time from curing an ailment to simply regaining strength.