Unhappy that his mother was nagging him, a full-time national serviceman punched her with a knuckleduster.

And when 19-year-old Rajen Zulueta Rajenthiran was given a two-year probation yesterday, Mrs Amelia Zulueta Rajenthiran, 48, and her husband had to post a $5,000 bond to ensure his good behaviour.

Rajen, who pleaded guilty to one count each of assault and being armed with an offensive instrument, must also perform 140 hours of community service.

The incident happened on June 27 after the teenager returned to his Yishun flat at around midnight with his girlfriend. Rajen's NS does not require him to stay in camp.

Mrs Amelia, who works as a patient-care assistant, was unhappy that his son's girlfriend had accompanied him home so late at night. When she followed them into his bedroom and nagged at him, Rajen became agitated and a dispute erupted.

Rajen told his mother to leave his room and threatened to punch her if she refused. When she did not leave, he punched her left arm.

He then picked up a knuckleduster from a table, slipped it onto his right hand and told his mother: "I will use this to punch you."

He then hit his mother twice on her left arm.

Mrs Amelia made a police report at around 1.30am that day. She later went to Khoo Teck Puat Hospital where a doctor found a bruise on her left arm.

According to court documents, the teenager claimed he had found the knuckleduster on a grass patch and taken it home.

Rajen's lawyer, Mr Peter Fernando, told the court in his mitigation plea that his client is extremely remorseful.

Mr Fernando pleaded with the court to give the teenager a chance to redeem himself and prove to his family that he is "not a lost cause".

For assault, Rajen could have been jailed for up to two years and fined up to $5,000.

And for being armed with an offensive instrument, he could have been jailed for up to two years.