“They're a bad influence on him.”
You have a teenage son. Recently he's started hanging out with a group of friends who are rough and like to get in trouble. You think that he's recently started smoking because of these friends. You think this to yourself.
They're a bad influence on him.
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(someone) is a bad influence on (someone)
If you're a "bad influence" on someone, it means that you cause or encourage them to do bad things, or make bad decisions.
People usually talk about someone being a "bad influence" on other people of the same age (especially young people) and the same social status.
"Bad influence" can be used in a couple of different ways:
Dolly was undoubtedly a bad influence on her youger sister.
Listening to that kind of music had a bad influence on them.
In addition to "bad", you can also "be" or "have":
a good influence
a huge influence
a positive influence
a negative influence