Answer:
The term "immune" is derived from a Latin term "immunis". It means "exempt from a service". In traditional sense in 15th century, it meant free from a liability or burden.
In 19th century, "immune" world also entered the medical/biological field. It started to represent the condition of "free from a disease" since a disease is also a liability or burden. Immune system thus came to be known as a system which helps us to stay away from diseases.