Tim Bowen sheds some light on the origins and definition of the phrase to get the sack.
Before the Industrial Revolution in Europe and the age of mass employment, people who needed work done and had the means to pay someone else to do it would hire workers with the skills to do specific jobs. These workers would carry the tools they needed for the job in a sack or in a bag made out of sacking. When the job was complete or when the employer no longer had any need for the services of a particular worker, he would hand him his sack. The worker would gather his tools together and put them in his sack and leave to find work elsewhere. Getting one’s sack back came to be associated with one’s services no longer being needed or being surplus to requirements. It has now passed into modern usage as an informal way of saying ‘to be dismissed’.