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2.
verb: nod off is what happens when your eyes and brain disobey you, and begin entering hibernation mode even though you know you should be up and awake. Nodding off happens most often when you sit in a comfortable seat in the back of the room during boring classes with droning old professors. Some people are gifted with the screensaver mode, which they can turn on to cover up the fact that they have nodded off
John Doe: "hey, don't nod-off now! you're in the first row for God's sake!"

Don Joe: "zzzzzzzzz grunt grunt zzzzzzzzz"
by Emperor Harry January 16, 2012
 
1.
to fall asleep usually unintentionally.
I often nod off in class.
The speech was so boring that several people in the audience nodded off before it was finished.
by oliverbr May 15, 2006