hello. Your surname is chinese. I have the same surname too. Our surname originates from the province of teochew in china. So your great grandfather would be a teochew. our people are spread all over the world due to early migrations to escape war, poverty, etc. my own grandfather emigrated to singapore during the 1920s. I believe ur grandfather might have left china earlier as there are alot of immigration down to south east asia much earlier than the 1920s. you can google teochew people if u're interested to know more. and our surname is indeed rare, even among teochew people. its ? in chinese. pronounce cheng in mandarin but in teochew its pronounce seng.because chinese words have various meaning depending on its usage, the word itself loosely translate to mean 'success'. anyway, really glad to meet u. i myself have nv met anyone outside of my family with the same surname either. would u happen to know your great grandfather's name? i could ask my grandmother if she knows him.