heavy chalk
n.— «Vegas has installed the San Antonio Spurs as prohibitive favorites, or “heavy chalk” in the industry parlance, to win their fourth title in eight years. The spread for Game 1 in San Antonio iniitally favored the Spurs by 8 points.» —“NBA Finals Game 1 Picks Offered At Mycapper.com” eMediaWire Jun 6, 2007. (source: Double-Tongued Dictionary)
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I know the word is yet to be researched, but my understanding is that it comes from the early days of horse racing.
At one point, the odds were written on chalk boards. Because the odds in horse-racing change depending on the betting, these numbers were often erased and re-written.
A heavy favorite would have it’s odds adjusted quite often, with lots of erasing and writing in chalk.
Thus the term ‘chalk’ as a favorite.
And not to pick a nit, but I don’t think “heavy” is part of this word. I’ve generally heard it as just “chalk” though occasionally modified with a word like “heavy” or “serious”.