1. beat a dead horse - Verb
2. beat a dead horse - Phrase
beat a dead horse (third-person singular simple present beats a dead horse, present participle beating a dead horse, simple past beat a dead horse, past participle beaten a dead horse or (colloquial) beat a dead horse)
(idiomatic) To persist or continue far beyond any purpose, interest or reason.
After having shown us three hours of instructional and safety videos, the inspector was simply beating a dead horse by telling us to buckle up as we got into the van.