Definition of nod off phrasal verb from the Oxford Advanced Learner's Dictionary (informal) to fall asleep for a short time while you are sitting in a chair I was practically nodding off in that meeting.

(idiomatic) To fall asleep, especially while in a seated position or in inappropriate circumstances. She can take Martha's place, trying to calm Howie down enough so he can nod off but keep beating on her the importance of keeping her mouth shut.