After the party, he was so well-oiled that he couldn't even fit the key into the lock.

well-oiled 是由副词 well（好地）和形容词 oiled（上油的）组成的复合形容词。oiled 是动词 oil（给...上油）的过去分词形式。 well-oiled 字面意思是"上好油的"，比喻机器运转顺畅。在这里用作俚语，形容人喝醉后身体不听使唤的状态，就像机器上了太多油反而失控一样。