毫米
háo mǐ · HSK 4
millimeter
📏 毫 = fine hair + 米 = meter; a hair-thin slice of a meter
毫 originally meant a fine hair, extended to mean a tiny fraction (one-thousandth). 米 here means meter, not rice. So 毫米 literally reads as thousandth-of-a-meter, which is exactly a millimeter.
米 — pictograph of rice grains separated by a thresher
米 is a pictograph showing grains of rice scattered around a central horizontal line, originally depicting kernels separated from the husk on a threshing surface. In oracle bone script the dots clearly represent individual grains. The meaning has stayed remarkably stable, referring to husked, uncooked rice and by extension other small grains.