There were quite a lot of people called Nassim in Singapore - and there is a Nassim Road, Nassim Hill, Nassim Park etc. They were sephardic Jews (their ancestors lived in Moorish Spain, until the Inquisition at the end of the 15th century). Later they settled in Syria, Egypt and Iraq. In the early 19th century, many of them moved East to Bombay, Calcutta, Singapore, Hong Kong, and Shanghai.
The name is probably the plural of 'nassi' meaning a village headman or minor authority.
If you have any more information on this previous owner of your cottage, please let me know - I'm a Nassim myself.