Robbie Dunne "opened his towel and shook himself" in front of fellow jockey Bryony Frost in the men's changing room, she said.
Frost was giving evidence at a disciplinary panel hearing claims she has been bullied and harassed by Dunne.
Dunne has said being naked was not a sexual act and could only recall being undressed in front of her when walking from the sauna to the weighing scales.
Frost, 26, wiped away tears at various stages of her evidence.
She recalled how Dunne had once appeared in front of her naked after leaving the sauna in the men's changing room, where female jockeys would sometimes go to collect kit.
"I remember Robbie through my amateur career being inappropriate to many of us," she said under cross-examination from Roderick Moore.
"I remember him opening his towel up and shaking himself, thinking it was funny."
Plans are in place to upgrade changing rooms, and Frost said they needed modernizing.
Frost has said she felt "isolated" after making bullying and harassment claims against Dunne.
The independent disciplinary panel, which follows charges from the British Horseracing Authority, heard several other female jockeys stated he had not been inappropriate to them.
"To me personally speaking the isolation I've found from speaking out, I wouldn't wish on anyone," she said.
"My opinion is they [other female jockeys] have a [riding] licence and I feel they are protecting themselves - and rightly so - to stay neutral," she said.
Witness statements were given to former BHA head of integrity Chris Watts by Bridget Andrews, Gina Andrews, Millie Wonnacott, Page Fuller, Lucy Pinchin and Lucy Barry.
Pinchin said Dunne "could be childish", while Fuller said Dunne had changed his behaviour from "persistent banter" towards her after she had confronted him and that now "they get on well".
Frost claimed to have seen fellow jockey Lucy Barry being "bent over a table" by Dunne - but Barry denied any such incident took place, Moore said.
Meanwhile, Lucy Gardner suggested Frost take direct action towards Dunne rather than complaining to the BHA.
"She actually said: 'You could just smack him' and I said I would definitely not do that," said Frost.
The hearing was earlier told Dunne threatened to cause Frost serious physical harm and also used "foul, sexually abusive and misogynistic language" towards her.
Dunne has said he "would never use the language she suggests", although he has admitted one breach of the rules by behaving in a violent or improper manner towards Frost at Southwell on 3 September 2020.
He denies similar charges at Stratford on 8 July and Uttoxeter on 17 August 2020, and also denies bullying and harassing her between 12 February and 3 September.
Dunne, 36, is accused of conduct prejudicial to the good reputation of horse racing.
Outlining his general behaviour towards her, she said other jockeys might tease her but he was "more aggressive, more direct" and singled her out.
"It wasn't just the odd occasion that he would take the mickey, as well as his manner towards me. It affected you more."
He mocked her in a tweet before the Virtual Grand National in April 2020, which she found to be a "personal attack in a very open way".
"He was always extremely calculated in whatever he said to me," said Frost.
"It was over a long period of time. It was never getting any better."
Frost said she had been left "really upset" and confused when Dunne "aggressively" criticised her riding at Stratford last year and accused her of cutting across him.
"I couldn't see where I had caused any interference," she said.
There followed an incident at Southwell in September when Dunne confronted Frost in the weighing room after he blamed her riding when his mount Cillian's Well suffered a fatal fall.
Reading from Dunne's witness statement, Moore said he had told her: "I will put her through a wing (side of the fence) as it was the only way she would learn because you are repeatedly doing this".
Frost said she believed him, which partly prompted her decision to contact the BHA.
"Dad always told me to stay quiet, don't start anything, just let it happen and move on. I used to think this but (with) the promise to hurt someone there's only so much you can take," she said.
Moore also quoted a witness statement from senior rider Tom Scudamore, who described the exchange as "nothing out of the ordinary."
"It was no different to what you have every third day. Otherwise I would have got involved," said Scudamore.
Frost said she used to share lifts to the races with Scudamore but he had not spoken to her since, until saying hello last week.
The hearing continues.
