(n.) a unit of work equal to approximately 1/3rd of a horsepower, or 250 watts
In the late 18th century, engineers looking to market their recent improvements in steam technology needed a means of demonstrating how their inventions compared to what was at the time the most widely used tool for heavy or demanding work—namely, a horse.
The solution was provided by the Scottish engineer and inventor James Watt.
He observed that a horse was able to lift a weight of 180lbs, which turned a 75ft-circumference millwheel 144 times in one hour. This meant the wheel travelled 181ft every minute—and given a pulling force of 180lbs, that enabled Watt to define the horse’s ‘power’ as just over 32,500 foot-pounds-per-minute (a figure that, for ease, he rounded up to 33,000).
One horsepower is ultimately the power required to match that of one horse—or, in Watt’s terms, the power needed to raise a weight of 33,000lb by 1ft (or 330lb by 100ft, or 33lb by 1,000ft) in one minute.
Watt’s calculations are nowadays considered somewhat inaccurate; a horse could scarcely be expected to pull with a 180lb force continuously, and as a result some contemporary analyses have suggested that his estimations were as much as 50% out. But Watt’s horsepower unit nevertheless remains a standard to this day. And, incidentally, it also provided the basis for a smaller unit.
How to talk about units of power less than 1 horsepower? Use a smaller creature, of course. Roughly 1/3rd of a single unit of horsepower—or roughly 250 watts—is now labelled one donkeypower.
Sounds like an internet joke, right? Wrong. This unit was actually first proposed way back in 1880s (albeit somewhat facetiously) to give engineers a workable unit to discuss machines of lesser power than the engines studied by Watt.