From Longman Dictionary of Contemporary Englishnod off phrasal verbSLEEPto begin to sleep, usually when you do not intend to and are sitting somewhere I missed the movie because I kept nodding off. → nod→ See Verb table
Examples from the Corpus
nod off• As the speaker droned on, only the occasional nudge from my husband kept me from nodding off.• Sarah had almost nodded off when Victor suddenly spoke. 