Utter the swear word "b****" and you are likely to get a dirty look. But the term could cost you your life in jail, as former Marine Zac found out.

The ex-military man, whose full name and age are being kept secret to protect him, was one of seven civilians who volunteered to go undercover as prison inmates for two months in the new American reality TV show, 60 Days In. The 12-part series airs on CI (StarHub TV Channel 403) tonight.

The other six participants - three men and three women - include a housewife, a security guard, a police officer, a teacher, an unemployed man and a social worker named Maryum Ali, who is the daughter of legendary boxer Muhammad Ali.

The show premiered in the United States last month. While ratings have not been disclosed, it has been renewed for a second season.

It was reportedly created to expose illegal activities and corruption within Indiana's Clark County Jail, where the seven volunteers were planted.

As Zac learnt during his 60 days behind bars, things can go bad quickly in a prison. After losing a bet with a fellow inmate, where the wager was to perform 20 push-ups, he did 21 and jokingly said: "That's 21, b****".

He realised his mistake when he saw the look on the inmate's face. Zac immediately apologised. Luckily for him, that was enough to get him out of trouble.

"In jail, I learnt that you don't trust anyone because everyone has done something to break the law." he told The Straits Times in a recent telephone interview.

The participants were sought out by Clark County Jail's sheriff Jamey Noel, who came up with the concept for the show.

On the show's debut episode, he said the idea stemmed from the desire to rid the facility of its rampant drug abuse and corruption problems. He said: "Before I took office, the inmates were running the facility. People were getting arrested on purpose because drugs were cheaper to get in jail."

He suspected that the prison staff were in cahoots with the inmates, which complicated the matter. The only way then, he figured, was to send in undercover people.

Footage for the show was filmed using hundreds of cameras hidden in the jail's ceilings and walls. No one else in the facility besides the sheriff knew the volunteers' real identities.

To prepare for their time on the show, the participants took lessons from law enforcement officials on prison politics and how to navigate the system without getting killed.

Zac was approached by Sheriff Noel after posting in online forums his views that prison inmates would benefit from a military-style boot-camp environment.

Zac was a combat engineer in the US Marine Corps Reserve and was posted to Afghanistan in 2010.

"The military is another form of institutional living and it's good because it gives you discipline. The jails and inmates in America would benefit from that."

Zac wants to become a policeman and thought the experience would give him a first-hand understanding of criminal psychology.

"I got the chance to see law enforcement from both sides of the picture and that will help me do a better job of helping the people I'm trying to incarcerate," said Zac, who is married with a baby boy.

"I realised that the majority of inmates I was with were not necessarily bad people. They just made bad choices and mistakes."

His wife, a secretary, had her reservations about him joining the show but eventually agreed that "it was a wise career choice" for him.

"My passion in life is helping other people. That's why I joined the Marine Corps and that's why I'm pursuing a job in law enforcement," he said.

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