(1):
(prep.) 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.
(2):
(prep.) Abreast; opposite to; facing; towards; as, against the mouth of a river; - in this sense often preceded by over.
(3):
(prep.) From an opposite direction so as to strike or come in contact with; in contact with; upon; as, hail beats against the roof.
(4):
(prep.) By of before the time that; in preparation for; so as to be ready for the time when.