means A unit of power, equal to about 746 watts, used to measure the output of engines and motors.

from Coined by the Scottish engineer James Watt in the late 18th century, who wanted a way to sell his steam engines to people who currently relied on horses. He estimated how much work a horse could do hauling loads in a mill and packaged that as one 'horsepower' — so a buyer could compare his machine directly against the animals he'd replace. His figure was generous (real horses can't sustain it), but it stuck, and we still measure car engines in the imagined labor of long-dead workhorses.