From Longman Dictionary of Contemporary Englishmoomoo /muː/ verb [intransitive] HBACif a cow moos, it makes a long low sound —moo noun [countable]→ See Verb table
Examples from the Corpus
moo• An outbreak of mooing in the crowd swells until most of the club is doing it.• In no time at all the kid was at my door with a bag full of sesame noodles and moo shu pork.Origin moo (1500-1600) From the sound 