It's the soccer trend that has infiltrated the Australian Open – the crowd screaming "siu" when they see something they like.
The only problem is, it sounds a lot like "boo".
And a part from creating confusion on that front, the rowdiness of the crowds this year is not impressing players.
World no. 2 in men's singles Daniil Medvedev, from Russia, and powerhouse doubles duo Nikola Mektic and Mate Pavic, from Croatia, have both said the crowds at Melbourne Park need to show more respect.
Australian tennis star Nick Kyrgios, who is a big advocate for atmosphere and hyping the crowd, first spoke of the "siu" trend on Tuesday night after his wild match at John Cain Arena.
"Yeah, it's just stupid. F***, I can't believe they did it so much," he said.
"They were doing some Ronaldo thing. (Cristiano) Ronaldo does it every time he scores. It's like — I thought they were going to do it for like 10 minutes.
"They did it for two-and-a-half hours, like every point. I don't know why, but I don't know. It was a zoo out there."
Andy Murray, who played before Kyrgios, said the yelling of "siu" was "incredibly irritating" and he initially thought he was being booed.
When Kyrgios took on Medvedev on Thursday night at Rod Laver Arena, the yelling of "siu" continued.
When news.com.au asked a bunch of fans who had been screaming it throughout the match why, they referenced Ronaldo. As to why it was here at the tennis this year, there was no answer other than it being a trend that caught on.
After another loud match, where Kyrgios swore at the crowd for screaming while he was trying to serve and blew up at the umpire for not controlling them, Medvedev told the arena to "show some respect" in his post-match interview on court.
In an awkward exchange, American tennis great Jim Courier tried to explain the crowd was not booing him but yelling "siu".
"So I think that is what is going on. I don't think they are booing you. I hope I am right on that," he said.
Medvedev then wrote "siuuuu" on the camera as he exited the arena.
Speaking to media after the match, Medvedev explained: "I was like, well, okay, I'm going to write it then because everybody is doing it. So I guess that's the thing to do.
"The only thing, between first and second serve, that's where, you know, it's tough.
"It's not good for the game I think to do it, because probably people don't know, but when you're getting ready for the second serve, well, it's a tough moment.
"I think, yeah, people should respect both players and just, you know, don't talk in these moments."
Some big names took to social media to criticise the trend.
AFL star Mason Cox commended Medvedev for calling out the crowd for their behaviour.
"Tennis is not soccer. When 76,000 people say it after a Ronaldo goal it's cool. When 100s yell it to a tennis star that hears it as "boo" it doesn't send the right message," he wrote.
American sports journalist Ben Rothenberg tweeted "This "siu" s**t is stupid and should stop", getting 2500 likes in support.
Earlier in the week, former Australian tennis player Roger Rasheed told SEN: "In Davis Cup ties we see the parochialism of the home country, we acknowledge that and respect that.
"This is entertaining to a level, and then I think it crosses a line where it's not a tennis match anymore because there's too much going on with all the punters there."
The "siu" yelling continued into Friday night.
It was especially prominent at Kia Arena as Australia's Dane Sweeny and Li Tu won against the Philippines' Treat Huey and Indonesia's Christopher Rungkat, and again as Kyrgios and Thanasi Kokkinakis knocked top seeds Mektic and Pavic out of the Australian Open.
At the former there was one point when the standard "Aussie Aussie Aussie, Oi Oi Oi" chant became "Aussie Aussie Aussie, Siu Siu Siu".
The tweets from around the world slamming the trend also continued.
Speaking in a press conference after the match about the rowdy crowd, Pavic said: "That's how they are here. We used to that … but it wouldn't hurt them to show some respect."
Australian world no. 1 in women's singles Ash Barty had a simple take on the use of "siu" at Melbourne Park.
"I don't really understand. It's not my thing," she said after her win against Italy's Camila Giorgi in straight sets 6-2 6-3 on Friday night.