disputant (n.)

c. 1300, "engage in argumentation or discussion," from Old French desputer (12c.) "dispute, fight over, contend for, discuss" and directly from Latin disputare "weigh, examine, discuss, argue, explain," from dis- "separately, apart" (see dis-) + putare "to count, consider," originally "to prune, make clean, clear up" (from PIE root *pau- (2) "to cut, strike, stamp").

The Latin word was used in Vulgate in sense of "to argue, contend with words." In English, transitive sense of "argue against, attempt to disprove, deny" is from 1510s. Related: Disputable; disputed; disputing.