From Longman Dictionary of Contemporary Englishbob your headbob your headHELLOYESto move your head down quickly as a way of showing respect, greeting someone, or agreeing with them He spoke to the girl, who bobbed her head. → bob
Examples from the Corpus
bob your head• She reddened and bobbed her head and sat down a little awkwardly.• A man turned into a baby, who bobbed his head and wept.• The birds just sort of bob their heads like they're wind up toys. 