Is it ensure, insure, or assure?
Old English cludig "rocky, hilly, full of cliffs;" see cloud (n.). Meaning "of the nature of clouds" is recorded from c.1300; meaning "full of clouds" is late 14c.; that of "not clear" is from 1580s. Figurative sense of "gloomy" is late 14c. Related: Cloudiness; cloudily.