hp is a short way to write horsepower, a unit that tells you how strong an engine is. Car adverts, motorbike specs, and old machine plates use it. You put it after a number: a 150 hp engine. One horsepower was meant to be about the work one horse could do, though actually an engineer set the number, and it is tidier than a real horse. It is a technical written form, not small-talk unless you like cars. People often say the full word horsepower, or just the number plus the Chinese unit, rather than saying the letters “H-P.”

馬力（講車子時口語也說「匹」）