When & where did the word "Christian" originate? The word "Christian" very rarely used in the new testament to refer to those that trust/believe on Jesus.

Mostly they were simply called "believers". 

When/where and by who coined the word "Christian"?
Actually, the word Christian is used three times in the New Testament. The first occasion answers your question (Acts 11:26c):


  It was at Antioch that the believers were first called Christians.


Regarding ''how'' they came to be called Christians, Wikipedia states:


  Kenneth Samuel Wuest holds that all three original New Testament verses' usages reflect a derisive element in the term Christian to refer to followers of Christ who did not acknowledge the emperor of Rome. The city of Antioch, where someone gave them the name Christians, had a reputation for coming up with such nicknames