Final answer:
CONTENTIOUS is best defined as quarrelsome, which indicates a tendency to engage in arguments or disputes.
Step-by-step explanation:
The word CONTENTIOUS most nearly means quarrelsome. This is evident from the context provided, where a person's natural combativeness is mentioned in relation to a heated discussion and is further exacerbated by alcohol consumption. The word signifies a tendency to engage in arguments or disputes, which aligns with the description of a person being quarrelsome.