warn someone off
- inform someone forcefully to stay at a distance
- to advise someone to refrain from some activities because of involved risks or other reasons
- to notify someone of staying away from danger ahead
- The board was placed near the manhole to warn off the kids from playing there.
- I had warned off Saima of her new friend because I knew he was just playing around her and would eventually get hurt.
- The fishermen had been warned off by the local authorities to not enter the sea as an upcoming cyclone had been forecasted by the weather department.
- The guards stood outside the door to warn people off until the fire was extinguished. The guards warned off everyone in the surrounding area.
- The police had warned off the residents to not open the door to unknown people as several incidents had been reported of robbers impersonating as officials and robbing the entire house.
Horse racing is known to be the origin of this idiom. Before the year 1969, the British Jockey Club had a rule empowering it to warn someone off the course, i.e. to ban someone who had broken Jockey Club regulations from riding or running horses at meetings under the club's jurisdiction.