1650s, "of or pertaining to the hair," from Latin capillaris "of hair," from capillus "hair" (of the head); perhaps related to caput "head" (but de Vaan finds this "difficult on the formal side" and "far from compelling, since capillus is a diminutive, and would mean 'little head', which hardly amounts to 'hair'"). Borrowed earlier as capillar (14c.). Meaning "taking place in capillary vessels" is from 1809. Capillary attraction attested from 1813. As a noun, "capillary blood vessel," from 1660s.