It takes a good deal of physical courage to ride a horse. This, however, I have. I get it at about forty cents a flask, and take it as required.

Leacock’s joke turns “courage” from a moral quality into a purchasable commodity. By claiming he buys his “physical courage” cheaply “at about forty cents a flask,” he implies that what passes for bravery in risky pursuits (like riding) is often just alcohol-fueled recklessness. The deadpan, matter-of-fact tone heightens the satire: he speaks as if courage were a household supply to be taken “as required.” The line also pokes fun at masculine ideals of hardiness and sporting daring, suggesting that such posturing can be chemically assisted and therefore hollow. The humor depends on the sudden shift from earnest observation to blunt confession.