What is the origin of the phrase "racing cert"?
Racing cert is from racing certainty and was once used literally when talking about horse racing but has since changed so it can be used to describe any (particularly sporting) dead cert.
Racing certainty
I found an antedating to the OED's 1859 of racing certainty, in The Monthly Magazine, Or, British Register, Volume XXIV of October 1837:
This is talking about horse racing.
Racing cert
Racing cert doesn't show up in print until much later, but that doesn't mean people hadn't already been using it in speech for some time. The earliest I found was in a 1920 snippet of St. George's Gazette, Volume 38 (Great Britain. Army. Northumberland Fusiliers):
On the 8th instant our first match against Washington resulted in a goalless draw after a strenuous game at Washington, and it seemed a "racing cert" on the game at home the following Wednesday, but we also ran, to the tune of being beaten by two goals to nil, our Washington visitors well deserving to win.
This is still sport, although rather than being used literally (about horse racing) it's changed to being used figuratively (about football).
Cert has been a slang abbreviation for certainty for a long time (1889 according to etymonline.com, which may or may not be true, but cert and dead cert are definitely not very recent).
Cert and dead cert are particularly popular among gamblers, who will often tell you with great conviction that they know of a cert in a given race or sporting event.
Meanwhile, racing certainty from the same demographic, exists both literally (if not reliably) of racehorses, and figuratively about other things.
Racing cert combines that phrase and that abbreviation. If somebody says something is a racing cert, they are sure it will be true, will come to pass, etc.
(That said, if when an actual racehorse is described as a racing cert, take that information with a grain of salt).
Here's a bit from the OED entry for racing certainty
n. Brit. A horse considered certain to win a race; (in extended use) anything regarded as certain to happen; a ‘sure thing’.
1859 Era 2 Oct. 3/4 All other ‘certainties’ we have a thorough distrust of, more especially of racing ‘certainties’.
And for cert Abbrev. of certain adj., certainty
n. slang A certainty (esp. in phr. a dead cert ); spec. in horse-racing, a horse that is considered certain to win.
1889 Man of the World 29 June 3/2 Love-in-Idleness is bound to take the Rous Memorial, and I hear Pioneer is a cert. for the St. James's.
And here's a chart to show that dead cert has always been far more common...
