In opposition to something, or not agreeing with it. It can also mean touching or pressed up to something, like leaning against a wall.
From Old English *ongean* “opposite, toward, against,” with the *-st* ending added later for emphasis. It shares roots with *again* in the idea of facing or turning toward.
‘Against’ started as a word about direction—facing something—and only later took on the sense of opposing it. Standing physically against a wall and standing morally against an idea both come from that same face-to-face stance.
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