prep.（介词）
(1)
In a direction or course opposite to:
逆：与某个方向或过程相反：
row against the current.
逆水行舟
(2)
So as to come into forcible contact with:
对撞：为与…达成猛烈接触：
waves dashing against the shore.
拍岸的波涛
(3)
In contact with so as to rest or press on:
倚在；紧靠着：与…接触为了休息或按压：
leaned against the tree.
斜靠着树
(4)
In hostile opposition or resistance to:
反抗；抵抗：处于敌对状况或与…相抗衡：
struggle against fate.
同命运抗争
(5)
Contrary to; opposed to:
与…相反；对：
against my better judgment.
与我的判断相反
(6)
In contrast or comparison with the setting or background of:
对照；对比：以…的布景或背景相对比：
dark colors against a fair skin.
深色衬托出白皙的肌肤
(7)
In preparation for; in anticipation of:
预期；以备：为…作准备；期待着…：
food stored against winter.
为过冬贮备的食物
(8)
As a defense or safeguard from:
预防；防御：以…为防御或保护对象：
protection against the cold.
预防感冒
(9)
To the account or debt of:
兑换；提取：作为帐上的应收款项或作为…的借项：
drew a check against my bank balance.
开支票支取自己的银行结余
(10)
Directly opposite to; facing.
正对着；面向
语源
--------------------------------------------------------------------------------
(1)
Middle English
中古英语
(2)
alteration of againes
againes的变化
(3)
from Old English ongeagn
源自 古英语 ongeagn
现代英汉词典
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against
[E5^eInst, E5^enst]
prep.
(1)
相反；逆着
against the law
违法
We sailed against the wind.
我们逆风行船。
Drug taking is against the law.
吸毒是违法的。
(2)
反对
We won our match against that team.
我们在比赛中赢了那个队。
No one is against this proposal.
没有人反对这个提议。
(3)
靠着，依着
to lean against the wall
靠着墙
(4)
与…成对照
against a dark background
与黑色背景成对照
The picture looks better against the light wall.
这幅画挂在浅色的墙上显得更美。
(5)
防备；防御
against the cold
防冷
against a rainy day
防备雨天
We are all taking medicine against the flu.
我们都在服药预防流感。
(6)
以…相抵
income against expenditure
以收抵支
参考资料：百度