Green’s Dictionary of Slang

cert n.

[abbr. SE certainty]
[late 19C+]

1. a definite winner, usu. in a sporting context.

2. a certainty.

3. attrib. use of sense 2.

In phrases

dead cert (n.) (also dead certainty) [SE dead, complete, utter]

[late 19C+] (orig. racing) an absolute certainty, esp. in race-course betting.

on a cert

[1920s] (Aus.) certain, guaranteed.