certainty (n.)

c. 1300, certeynte, "surety, pledge," from Anglo-French certeinté (late 13c.), Old French certainete "certainty," from Latin or Vulgar Latin *certanitatem (source of Old Spanish certanedad), from Vulgar Latin *certanus (see certain). Meaning "that which is certain, a clear fact or truth" is attested from early 14c.; meaning "quality or fact of being certain; full assurance of mind, exemption from doubt" is from early 15c.