Displeased with his neighbour over issues such as staring incidents, an elderly lorry attendant smeared his own faeces on the younger man's letterbox in July.
Sim Ann Heng, 71, harassed retiree Tan Yam Chwee, 67, again later that month by pasting an obituary photo on Mr Tan's letterbox.
Sim was jailed for two weeks last Friday after pleading guilty to two harassment charges.
A third harassment charge was considered during sentencing.
Prior to these incidents, Sim was convicted on June 19 of assaulting Mr Tan and was sentenced to five months' jail.
Court documents did not reveal details about this case.
Sim later filed an appeal against his sentence and was released on bail.
Still unhappy with Mr Tan, who lived two storeys above him in a housing block in Commonwealth Avenue, Sim went to the ground floor some time between 6.30pm on July 10 and 7.50am the next day and smeared his faeces on the retiree's letterbox.
Mr Tan alerted the police after spotting the mess and town council workers later cleaned it up.
After this incident, the police installed a camera in the area, and an officer was making his rounds at around 1.10am on July 15 when he spotted a man writing something on a letterbox.
Investigations revealed that Sim had taken an obituary picture of a dead man, wrote Mr Tan's name on the cutout and pasted it on the letterbox.
When more officers arrived at the scene, Sim admitted that he had pasted the picture there as he was unhappy with Mr Tan.
The court heard that Sim withdrew his appeal on Aug 13 and started serving his sentence for the assault offence later that day.
For each count of harassment, he could have been jailed for up to six months and fined up to $5,000.