I deny your contradiction and raise you an opposed dispute.

gainsay (gain·say): / GAYN say /
transitive verb

(1) to deny or dispute; to declare to be invalid or untrue
(2) to oppose or contradict; to act or speak against

[From Middle English gainsaien. Coined from Middle English gain- (“against,” from Old English gegn) + saien (“to say”).]

Usage: I wouldn’t gainsay the professor unless you’re absolutely certain your position is correct.