Joe Wicks appeared to turn the air blue during one of his fitness class as he expressed his exhaustion over a very vigorous workout.
The Body Coach became short of breath after performing a move that saw him 'zooming' across the room multiple times and before launching into a press up.
Joe, 33, finished the sweaty routine doubled up in pain - and fans think he let out a rude word.
Many took to social media to check if he had said: "That's f***ed with me, that has."
The video has been watched over 340,000 times since it premiered live on YouTube on Monday.
After watching the clip again, some other viewers protested Joe's innocence and said he simply cried out: "That's buckled me, that has."
While others joked they'd uttered far worse things while trying to take part in one of Joe's online fitness classes.
Joe has been one of the 'winners' of the government enforced lockdown after gaining scores of new fans with his online workout classes.
When schools were shut back in March, Joe vowed to become the nation's PE teacher and started teaching lessons via his YouTube channel.
After seeing his viewing figures rocket, Joe launched several spin-off ideas so that adults can get involved too.
His '7 Days of Sweat' challenge has proved particularly popular, which sees fans copy 20 minute work-outs for seven consecutive days.
But the father-of-two has confirmed he's not raking in all the profits from his channel and has turned down offers from the likes of Channel 4 and the BBC to broadcast through them.
By keeping his content strictly to his YouTube page Joe has ensured fans around the world can watch him for free whenever they wish.
He is also donating all his advertisement revenue to the NHS.
"It’s a phenomenal amount of money and it just felt right, seeing as it was the community that built it. I didn’t want people to think for a second that I had the intention of financially benefiting from any of this," Joe told Men's Health magazine.
