The Sydney Roosters prop admitted to the judiciary on Monday night he told Mam to "f*** up you monkey" during a verbal stoush in the Las Vegas season-opening double-header against Brisbane.
But in a 90-minute hearing at the NRL's Sydney headquarters, Leniu also said he did not know the racial connotations of the term and only learned of them the following morning.
He said players of colour in NRL squads regularly called each other terms such as "blacky", "monkey" and "black c***", and therefore he did not realise it would be offensive.
"At the time I thought it was one brown man saying something to another brown man," Leniu told the panel of Sean Hampstead, Bob Lindner and Geoff Bellew.
"The use of words is so common.
"This game happened so fast and in that split second I said a word, I didn't know any meaning to it.
"I didn't know how much that meant to the Indigenous community and his family.
"It was just one of those things. I tackled someone, they said something to me and I said something to them."
Mam wrote in a statement to the panel that he saw red after the incident, and was "angry" and "disappointed".
Leniu said he was extremely apologetic to Mam, and still wanted to fly north to make peace with the Broncos five-eighth and his family once the Indigenous star was ready.
"There is no room for racism in this game," Leniu said.
"I'm glad he brought this thing up. I had no racial intent towards Ezra and the Indigenous community.
"I love them and their culture. I don't think there'd be a game without those people."
Leniu said he was not aware Mam was Indigenous until after the match, and that he regretted a hotel corridor verbal altercation with Broncos player Pat Carrigan.
He also said post-match comments in which he labelled the incident "banter" came because of a lack of knowledge around the term, and that he was only made aware of the history around the word "monkey" when contacted by an Indigenous woman the next morning.
In an embarrassing blunder for the NRL, counsel Lachlan Gyles referred to Samoan Leniu as Tongan, and referred to him as "Spencer Luai" at the beginning of the hearing.
Gyles asked Leniu if he was aware of incidents such as those involving Adam Goodes at the Sydney Swans, or other racial incidents involving references to monkeys or apes.
When Leniu said he was not, Gyles suggested to Leniu that "most people who have been in Australia for 15 years would know it would ordinarily be racism to call someone a monkey".
"It is beyond argument that in Australia in 2024, calling an Indigenous person a monkey does constitute racial abuse, and racial abuse of a very serious nature," Gyles said.
Leniu's lawyer James McLeod pushed for a ban of four games, comparing the case to Marcelo Montoya's homophobic slur directed at North Queensland winger Kyle Feldt in 2022.
But Gyles said this case warranted a ban of double that length, arguing the case was different because Mam was part of a minority group and offended by the slur.
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