The first recorded use of this phrase is from the fourteenth century. In medieval times, the peasantry were not allowed to cut down trees, but they were permitted to gather firewood from loose or dead branches using a hook (bill-hook) or a crook (a staff with a curved end like those used by shepherds). No doubt desperate peasants often exceeded the strict use of these tools, and so the term has evolved into its current usage, meaning to achieve something by whatever means possible.