毛 "hair" + 细 "thin": fine as a hair, the same image as English capillary, from Latin capillus "hair". It mostly appears inside terms: 毛细血管 capillary vessels, 毛细管 capillary tube, 毛细作用 capillary action.
culture
At a 兰州牛肉面 shop the noodles are pulled to order in the width you ask for. 毛细 is the thinnest; others include 二细, the flat 韭叶 "chive leaf" and the belt-wide 大宽.
