Here is the story as I know it. Years ago, craftsmen, such as carpenters, lived with their employers. They brought their tools with them in a sack when they started the new job. The employer usually kept the tools in his workroom for safe keeping. However, if the craftsman was discharged from his job, he was given back his sack full of tools. He "got the sack"! Hope this helps

To be sacked is to be fired from a job. To get the sack has the same meaning. I couldn't find any history behind this usage. Here is an excerpt from Wikipedia. "Firing" is a common colloquial term in the English language (particularly used in the U.S.) for termination. The term "firing" may have been initiated[2] in the 1910s at the National Cash Register Company. Other terms for dismissal are being "sacked", "canned", "let go", "ran-off", "axed", "given walking papers", "given the pink slip" or "boned". Other terms, more often used in Commonwealth countries, include "to get the boot" and "to get the sack".[3][4]

Naomi is absolutely correct.