1708, "curl-like fringe or tuft," from Latin cirrus "a lock of hair, tendril, curl, ringlet of hair; the fringe of a garment." In meteorology, cirrus clouds attested from 1803. So called from fancied resemblance of shape.
Plural cirri (sîr'ī')
A high-altitude cloud composed of feathery white patches or bands of ice crystals. Cirrus clouds generally form between 6,100 and 12,200 m (20,000 and 40,000 ft). See illustration at cloud.