Sian is a term used to show displeasure or extreme boredom. So calling someone by that word could mean a boring person.
What Does "Sian" Mean In English?
Well Sian is also a name it is a Welsh name that means gods gracious gift it is also my name
Sian is not English dictionary word. It can be his own created word or it can be a slang word in some language. You should ask your friend what does this really mean for him.