›in opposition to; opposed to: I know you’d like to get a more expensive car, but I’m against it.It’s against the law to throw your trash there (= It’s illegal).She voted against the tax increase.He warned them against repeating (= not to repeat) the mistakes of the former administration.›Against also means in competition with: He would have to run against O’Toole for county treasurer.›To go against something means to go in the opposite direction to it: swimming against the current