His performance was poor, so he got the sack from his last job.

the 是定冠词，在这里特指"解雇"这件事。 sack 本义是"麻袋"，在俚语中表示"解雇"，源自旧时工人被解雇时需收拾工具装进麻袋离开的习俗。 the sack 是固定搭配，表示"被解雇"的意思。