against

Từ điển Oxford Advanced Learner 8th

against BrE [əˈɡenst] NAmE [əˈɡenst] BrE [əˈɡeɪnst] NAmE [əˈɡeɪnst] preposition
 For the special uses of against in phrasal verbs, look at the entries for the verbs. For example count against sb is in the phrasal verb section at count.
1. opposing or disagreeing with sb/sth
•the fight against terrorism
• We're playing against the league champions next week.
• We were rowing against the current.
•That's against the law.
•She was forced to marry against her will.
•Are you for or against the death penalty?
•She is against seeing (= does not want to see) him.
•I'd advise you against doing that.
2. not to the advantage or favour of sb/sth
• The evidence is against him.
• Her age is against her.
compare ↑for _prep. (⁷)
3. close to, touching or hitting sb/sth
•Put the piano there, against the wall.
•The rain beat against the windows.
4. in order to prevent sth from happening or to reduce the damage caused by sth
•an injection against rabies
•They took precautions against fire.
•Are we insured against theft?
5. with sth in the background, as a contrast
•His red clothes stood out clearly against the snow.
• (figurative)The love story unfolds against a background of civil war.
6. used when you are comparing two things
•You must weigh the benefits against the cost.
•Check your receipts against the statement.
•What's the rate of exchange against the dollar?
see as against sth at ↑as _conj., the cards/odds are stacked against you at ↑stacked

Word Origin:
[against] Middle English: from ↑again + -s (adverbial genitive) + -t probably by association with superlatives (as in amongst).