means A rough sheet of paper or cloth coated with abrasive grit, used to smooth or scour surfaces by friction.

from A transparent English compound: sand + paper, naming exactly what the earliest versions were — paper coated with glued-on sand or crushed glass to make a roughened, grinding surface. The word arrived in the 19th century alongside the manufactured product; before that, woodworkers reached for shark skin, dried horsetail plants, or actual sand on a backing. Modern 'sandpaper' rarely contains sand at all — it's usually aluminum oxide or garnet — but the honest old name stuck.