nod off

to begin sleeping, especially not intentionally

打瞌睡;(尤指无意中)睡着

我们都知道nod是点头的意思，回忆一下我们上课时犯困又不能立马躺下睡觉的样子，是不是时不时得点个头。这里nod off就指不是故意的情况下睡着的状态。

After our busy day, we both sat and nodded off in front of the TV.

忙碌了一天，我们两个都坐在电视机前打起了瞌睡。

The student appeared to nod off in the classroom yesterday.

那个学生昨天在教室里似乎睡着了。