to be hostile to or against; to resist
to fight or compete with
to be hostile to or against (somebody or something); to offer resistance to (them or it)
to place (something) opposite or against something else, e.g. as a contrast or counterbalance so as to provide a counterbalance, contrast, etc to it
opposer noun
opposing adj
[French opposer from Latin opponere, from op- + ponere to place]