Merriam-Webster's Collegiate® Dictionary, 11th Edition
AGAINST
I. preposition
Etymology: Middle English, alteration of againes, from again
Date: 13th century
1. in opposition or hostility to , contrary to , in competition with , as a basis for disapproval of ,
2. directly opposite ; facing , exposed to, compared or contrasted with ,
4. in preparation or provision for , as a defense or protection from ,
5. in the direction of and into contact with , in contact with , in a direction opposite to the motion or course of ; counter to ,
7. as a counterbalance to , in exchange for , as a charge on , before the background of ,
II. conjunction
Date: 14th century
in preparation for the time when