set on somebody phrasal verb British English ( see also ↑ set )
1 . set somebody/something on somebody to make people or animals attack someone:
The farmer threatened to set his dogs on us.
2 . [usually passive] if you are set on by people or animals, you are suddenly attacked by them:
A thirty-five-year-old man was set on by four youths last night.
3 . set somebody on/onto somebody to give someone information about a person who you think has done something wrong, because you want that person to be found and caught:
If I refuse, he’ll set the police onto me.