The definition of against is someone or something with which you disagree.(preposition)
To vote “No” on a proposal is an example of voting against the proposal.
Against means something that is going the other way.(preposition)
Walking north into the wind that is blowing south is an example of walking against the wind.
See against in Webster's New World College Dictionary
Origin: ME ayeynst, opposite to, facing < OE ongegn, ongean (see again), with adv. gen. -es + unhistoric -t
See against in American Heritage Dictionary 4
Origin: Middle English
Origin: , alteration of againes
Origin: , from Old English ongeagn.
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