British people often express a probability as "betting odds". In Britain, and some other countries, it is legal to place bets on whether some future event will happen. The event could be a particular horse winning a race, a football team winning a match, etc. Events are not restricted to sporting activities*. You can bet on. e.g. whether snow will fall somewhere in England on a certain date.
You can ask an individual or company specialising in taking bets (often called "bookmakers") to "give you odds" on the event happening. If they decide they want to take the bet, they quote ("give") you "odds", a figure, expressed as [number] to [number], such as "three to one", "seven to four", etc. These can be written as 3/1 and 7/4. They are the ratio of the amount of money you will win (the "winnings") to the amount you pay to place the bet (the"stake"). You also get the stake back if you win. Bookmakers advertise odds on popular events.
The odds reflect how likely the bookmaker thinks an event is to happen. An unlikely event will get bigger ("longer") odds than a likely one. Thus you might get odds of 1000 to 1 that it will snow in England on August 1st, but 2 to 1 that a well-regarded horse and jockey will win a particular race. Very "short" odds, often less than 2 to 1, means the bookmaker thinks the event is very likely. In the example given, the odds are 7/4, which is the same as 1.75 to 1. Odds are always given as whole numbers. If a bookmaker thinks an event is exceedingly likely to happen, so much so that they don't want to take bets, they will give bets where the ratio of winnings to stake is less than 1 to 1, e.g. three to five (3/5). These are called "on" odds, and described as "three to five on". You would have to bet £5 to get £3 back (plus your stake). An "odds-on favourite" is considered sure to win a race. The expression means something like the American "slam-dunk".
So, the narrator "I" is saying that, based on his understanding of the meaning of the look that Will gave to Louise, something is very likely. Could this be that Will is attracted to Louise?
*In 1964, a British man asked a bookmaking company for odds on whether a human would stand on the moon before January 1971. He was quoted 1000 to 1, and placed a bet of ten UK pounds, equivalent to around $223 in present values. On July 21, 1969, he was paid his winnings live on TV, after Neil Armstrong landed. Now his winnings would be equivalent to around $250,000.
Best Answer
Yes this is idiomatic!
You often hear this used in the exact way you have in your question and it's pretty well established (at least in BrE).
It's worth noting that it's sometimes used to refer to ones house as well, even though this isn't the true definition.
E.g. Someone might say:
This sentence should really be:
Clearly they mean back at the house, rather than in the home country! (Even though they will no doubt have bread "back home").