Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.

WORD ORIGIN

1520s, "certain," from cock (n.1) + sure (adj.). Probably "as assured as a cock." "The word was originally perfectly dignified, and habitually used in the most solemn connexions" [OED].

Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.