The Collaborative International Dictionary of English v.0.48:
Against \A*gainst"\ (?; 277), prep. [OE. agens, ageynes, AS.
ongegn. The s is adverbial, orig. a genitive ending. See
1. Abreast; opposite to; facing; towards; as, against the
mouth of a river; -- in this sense often preceded by over.
Jacob saw the angels of God come against him.
2. From an opposite direction so as to strike or come in
contact with; in contact with; upon; as, hail beats
against the roof.
3. In opposition to, whether the opposition is of sentiment
or of action; on the other side; counter to; in
contrariety to; hence, adverse to; as, against reason;
against law; to run a race against time.
The gate would have been shut against her.
An argument against the use of steam. --Tyndale.
4. By of before the time that; in preparation for; so as to
be ready for the time when. [Archaic or Dial.]
Urijah the priest made it, against King Ahaz came
from Damascus. --2 Kings xvi.
Against the sun, in a direction contrary to that in which
the sun appears to move.