Against As A Preposition

Against means not in favour of

• There were twenty votes against the proposal and only fourteen in favour of it, so it was abandoned.
• We rowed hard against the current, but made little progress.

Against can also mean hitting or touching.

• I hit my head against a wall and hurt myself.
• I left the ladder leaning against a wall.
• The waves beat against the cliffs.
• The piano stood against the wall.