1
general::
Phrase(s): nod off
to fall asleep, usually while sitting up. • Jack nodded off during the minister’s sermon. • Father always nods off after Sunday lunch.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Fall asleep momentarily, doze, as in Grandma spends a lot of time in her rocking chair, nodding off now and then. This expression alludes to the quick involuntary dropping of one's head from an upright position when drowsy or napping. The verb nod alone was so used from the mid-1500s. Also see DROP OFF, def. 1.
American Heritage Idioms