"Horsepower goes back to the days when horses ruled the world, literally!
The Scottish Engineer and inventor, James Watt (1736-1819), introduced the
term "horsepower". The term was and is used to indicate the rate at which
an engine can deliver work. As such, it is a measure of power, that is,
energy produced or worked done by an engine per unit time.
Mr. Watt made considerable improvements to steam engines (which were
invented about 70 years earlier, in 1698, by Thomas Savery). His
improvements led to engines that were four times more efficient (used less
coal) than others, much smaller than before, and much more powerful (from
earlier 6 horsepower ones to about 200 horsepower). Oh, you could NOT fit
one under the hood, any hood).
Naturally, Mr. Watt wanted to tell how powerful his engines were. So,
after some tests (not with engines but with horses) he established that on
the average, a horse could haul coal at the rate of 22,000 lb-ft per
min. For some reason, unknown to me, he decided the raise this number by
50% to arrive at 33,000 lb-ft per minutes (No, horses those days were not
on drugs; steroids were not known at that time, but I am sure Mr. Watt had
his own reasons for this increase).
So, if an engine can push 33,000 Lb of something one foot in one minute, we
say that is a one-horsepower engine. By the way, I believe that deliverable
power, also known as brake or shaft power, is the one used in automobile
industry in the US, and this indicates the practical ability of the engine,
i.e., engine power minus losses due to friction, compression, heat, etc."