Prendre ses jambes à son cou translates as running away quickly.
Word-for-word, prendre ses jambes à son cou means ‘to take the legs to one’s neck’.
This expression first emerged in 1618 as prendre ses jambes sur son col – to take the legs on one’s neck or collar – meaning to decide to go on a trip.
This meaning came from the idea that you would need to pack extra belongings to take with you if you went away.
At that time belongings would be carried in a cross-body bag with the weight resting on the side of the neck.
The modern usage, however, has a more pressing meaning.
If someone has to prendre les jambes à son cou they must flee, make a hasty escape or even take to their heels and run away.
It is a very visual phrase and conjures an image of someone running with their legs pumping high to get away as quickly as possible.