against (prep.) Look up against at Dictionary.com
12c., agenes "in opposition to, adverse, hostile; in an opposite direction or position, in contact with, in front of, so as to meet," originally a southern variant of agan (prep.) "again" (see again), with adverbial genitive. The parasitic -t turned up mid-14c. and was standard by early 16c., perhaps from influence of superlatives (see amidst). Use as a conjunction, "against the time that," hence "before," is now archaic or obsolete.