Prendre ses jambes à son cou

Literally: To take one's legs around one's neck

Example: “En voyant le chien arriver, le voleur a pris ses jambes à son cou.” — When he saw the dog coming, the thief ran off as fast as he could.

English equivalent: To take to one's heels

Origin: The 17th-century lexicographer Furetière records the earlier form prendre les jambes sur son col — to take one's legs onto one's collar — from the practice of carrying a travel bag slung by its strap across the neck and shoulder. Reminding someone leaving in a hurry not to forget their jambes, their legs, alongside the rest of their luggage was a small joke about not forgetting the one thing you'd actually need if the journey turned into a run. Col softened into the modern cou by the 18th century, and the phrase kept only the running.