Get the sack
The usual explanation for the origin of this expression is that working people used to provide their own tools, which were carried in a bag or sack. When they were given their marching orders, they picked up the sack and went on their way. From this explanation, one would expect the expression to be very old, at least from The Middle Ages, but it is not. In French, donner le sac, to give the sack, has been in use since the 17th century, but its first citation in English, according to the OED is only from 1841. By this time, many jobs from which people could get the sack did not involve a sack or bag of tools at all. This casts some doubt on the explanation of its origin but until more evidence comes to light it remains the only contender, if slightly dubious.