colloquial abbreviation of certainty, attested by 1889 (in dead cert). Cert (adv.) "forsooth, indeed," was in Middle English, from Old French, from Latin certo, certe, but it became obsolete or dialectal.
Entries linking to cert
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).
The meaning "that which is certain, a clear fact or truth" is attested from early 14c.; the meaning "quality or fact of being certain; full assurance of mind, exemption from doubt" is from early 15c.