set somebody/something against somebody/something phrasal verb ( see also ↑ set )
1 . to make someone start to fight or quarrel with another person, especially a person who they had friendly relations with before:
The bitter civil war set brother against brother.
2 . set yourself against (doing) something to decide that you are opposed to doing or having something:
She’s set herself against going to university.
3 . set something against tax to officially record the money you have spent on something connected with your job, in order to reduce the amount of tax you have to pay