The phrase "get the sack" means to be dismissed or fired from a job. The origin of this phrase is believed to date back to the 17th and 18th centuries in Europe, particularly in England. 

In earlier times, manual laborers and tradesmen would often carry their tools and personal belongings in a sack. When a worker was dismissed from a job, they would be given their sack to pack up their tools and leave. Thus, "getting the sack" became synonymous with being fired or let go from employment. 

Another theory suggests that the phrase may have origins in the French language. The French expression "mettre à la porte" (literally "put at the door") means to dismiss someone. The term "sacquer," which means to expel or dismiss, may have influenced the English phrase. French workers might have used "sac" (sack) in a similar context.