What does against mean?
The definition of against
A*gainst”, prep.
Etym: [OE. agens, ageynes, AS. ongegn. The s is adverbial, orig. a genitive ending. See Again.]

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. Tyndale.
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. Fielding. 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. 11. Against the sun, in a direction contrary to that in which the sun appears to move.