The term “a fortiori” is a Latin term that means “with a stronger reason,” “all the more reason,” “even more certainly.” It refers to a logical or rhetorical technique where one concludes that if something is true in a stronger or more difficult case, it is certainly true in a weaker or less difficult case.
Examples:
“If the law prohibits driving at 100 mph in a school zone, a fortiori, it prohibits driving at 120 mph.”
“If John was too drunk to operate a motor vehicle, a fortiori, he was too drunk to fly an airplane.”