1 a : in opposition to (someone or something) ▪ Everyone was against them. [=everyone opposed them] ▪ We must continue the struggle for justice and against injustice. ▪ She voted against the proposal. ▪ You're either for/with me or against me! [=you either support me or oppose me] ▪ He spoke against appeasing the enemy. ▪ The U.S. fought against Germany in World Wars I and II. = The U.S. and Germany fought against each other in World Wars I and II. ▪ Some people were for the proposal but others were against it. ▪ There's a law against doing that. ▪ His parents were angry when they learned he had borrowed the car against their wishes. [=he borrowed the car even though they did not want him to] ▪ Against my advice, she quit her job. [=she quit her job even though I advised her not to] ▪ He was being held against his will. [=he was being kept in a place by force even though he wanted to leave] b : in competition with (someone or something) ▪ two runners racing against each other ▪ It's the Yankees against [=versus] the Red Sox tonight.
2 —used to indicate the person or thing that is affected or harmed by something ▪ He denies the charges that have been made against him. ▪ The evidence against him is very convincing. ▪ The war was a crime against humanity. ▪ Someone has been making threats against her.
3 : not agreeing with or allowed by (something, such as a law) ▪ You can't do that. It's against the law. [=the law does not allow it] ▪ Touching the ball with your hands is against the rules. ▪ I won't do it. It's against my principles. [=my principles don't allow it; I believe that it is wrong] ▪ They were acting against tradition. [=acting in a way that did not agree with tradition]
4 a : not helping or favoring (someone) ▪ Everything was/went against them. [=everything that happened was unfavorable to them] ▪ Her appearance is/counts against her. [=her appearance does not help her; people form a bad opinion of her because of her appearance] b : as a reason for disliking (someone or something) ▪ I have nothing against him. [=I don't dislike him; I have no reason for disliking him] ▪ We disagree about many things, but I don't hold it/that against you. [=I don't dislike you because of it/that]
5 —used to say that one thing is being compared with another ▪ Profits are up this year against last year. [=profits are higher this year than they were last year] ▪ The yen rose/fell against the dollar. ▪ a graph that plots height against weight ▪ We will weigh/balance the risk against the possible benefit. [=we will compare the risk with the possible benefit]
6 a : as a defense or protection from (something) ▪ He built a crude hut as a shelter against [=from] the cold. ▪ injections against flu b : in preparation for (something) ▪ saving against [=for] an uncertain future
7 —used to describe hitting or touching something or someone ▪ I accidentally knocked/hit my head against the shelf. ▪ The waves were beating against [=on] the shore. ▪ She accidentally brushed against him as she walked by. ▪ The ladder was leaning against the wall. ▪ She was resting her head against the side of the chair.
8 : in a direction opposite to the movement of (something) ▪ They were sailing against the wind. ▪ swimming against the current/tide
9 : with the background of (something) ▪ The tree looked beautiful when viewed against the dark sky. ▪ Her colorful coat stood out against the dull colors of the room.
10 : as a charge on (something) ▪ The purchase was charged against [=to] her account. [=the amount of money needed for the purchase was added to the amount owed in her account]