Targum Jonathan on Deuteronomy
(as for example) if any one goeth with his neighbour into the thicket to cut wood, and he driveth his hand with the axe to cut wood, and the iron flieth apart from the haft and lighteth on his neighbour that be die, he may flee to one of those appointed cities, and. save his life.
Targum Jerusalem
He who may go with his neighbour into the thicket to cut wood, and exerting himself with the axe to cut the wood, the iron separate from the handle, and fall upon his neighbour that he die, he may flee into one of those cities and live.