in opposition or hostility to (something)
the rule against smoking
unfavourable to (somebody)
His appearance is against him
as a defence or protection from (something)
She warned them against opening the box
contrasted with (something)
silhouetted against the sky
in the direction of and in contact with (something)
Rain beat against the windows
in a direction opposite to the motion or course of (something)
We swam against the tide
in preparation or provision for (something)
saving against his retirement
with respect to (something)
customs that had the force of law against both parties
in exchange for (something)
as against
compared with (something else)
have something against
to have a reason for disliking (somebody)
[Middle English, alteration of againes, from again]