The story
From ante- + post, the second component in the sense of the traditional "betting post", a stake set up at a racecourse to signal the beginning of fixed betting.
Etymology from Wiktionary, CC BY-SA 4.0.
antepost
Today it means: Being a kind of bet, in horse-racing and greyhound racing, placed before the racecourse's betting market has opened, on the expectation that the price of the animal is presently more favourable than it will be when the market opens. Full definition →