According to the New Penguin English Dictionary (1986), to air cool means to cool the cylinders of an internal combustion engine directly with air.
The “of an internal combustion engine” part is in parenthesis. I don’t know if that means that it’s an optional part of the definition or if the dictionary is trying to tell me, “of course, everyone halfway intelligent person knows that we’re talking about internal combustion engines, but for those few idiots, such as yourself, who don’t know that we decided to put in this parenthetical comment.”
It doesn’t take a brilliant mind to figure out that if you cool something with air then you are air cooling it. Duh.
But why is it only cylinders that air cool applies to? Shouldn’t it be anything that is cooled with air? If I decide I want to take take a warm beer outside on a frigid winter day and hold it in the air for a while until it cools down, would I be incorrect to say that I want to air cool the beer?
I’m not much of a beer drinker, but I think I’m going to try that when winter arrives and then rub some lexicographer’s nose in the beer.