LAS VEGAS — People are putting their money where Conor McGregor’s mouth is.

Though McGregor isn’t expected to leave his first professional boxing match a winner this weekend, if the brash UFC lightweight champ is somehow victorious, Floyd Mayweather Jr. won’t be the biggest loser in Las Vegas.

Despite the overwhelming consensus from experts that the 29-year-old Irishman has little chance to beat the undefeated boxing legend this weekend, bettors are overwhelmingly in McGregor’s corner — accounting for 89 percent of all action, according to BetOnline.ag — and could be setting the city’s sportsbooks up for one of its darkest days ever.

“This could be the worst loss in the history of MGM Resorts,” Jay Rood, vice president of MGM Resorts Race and Sports, told ESPN.

Before the bout was officially scheduled, the Westgate Las Vegas Superbook listed Mayweather as a -2500 favorite, and McGregor as a +1100 underdog. As recently as June, Mayweather — who returns from a two-year layoff — was listed at -800, with McGregor at +500.

With the action continuing to be so one-sided in McGregor’s favor, the fighter who has never lost to a professional boxer in 49 fights is currently listed at -475 to win, while the mixed martial artist is at +325 to win.

Of Mayweather’s eight fights since 2011, the 40-year-old has only had such low odds to win in two fights — against Manny Pacquiao and Canelo Alvarez.

“I’ve never seen anything like this. I can’t believe it’s this lopsided,” Zach Jornlin, sportsbook supervisor at MGM Resorts, told The Post. “There are over 6,000 more tickets for McGregor than there are for Mayweather. … A lot of people hate Mayweather and a lot of people want to see him get knocked out, and make some money. This would be a really, really bad loss for us.”

“The whole world is betting McGregor, and [the line] just keeps dropping and dropping.”

Sportsbooks are hoping that “sharps” — expert gamblers — will provide a safety net, by placing heavy bets on Mayweather later in the week, when the line falls even more, and provides its greatest value for the favorite.

“I think we’re going to see more from Mayweather this week, and it should help even it out a little bit more, but we may keep seeing more for McGregor, too,” Jornlin said. “Hopefully we have some big bets on Mayweather coming this weekend.”