If you're argumentative, you have a tendency to quarrel or squabble. An argumentative classmate always finds a reason to disagree with the teacher's viewpoint.
You'd probably enjoy being on a debate team if you're naturally argumentative. Argumentative people always seem to come up with an argument or difference of opinion. When it first appeared in the 15th century, argumentative simply described anything having to do with arguments, from the Latin root arguere, "make clear, make known, or demonstrate." By the 1660s, it had also come to mean "fond of arguing."