What is meaning of "Kattar Hindu"?

A person who is Hindu by birth / belief & faith. 

The term Kattar is usually found in some other religions who do not permit any deviations. 

Kattar basically denotes an Ultra Orthodox, Die-Hard Religious Fundamentalist Person. 

One who strongly believes in his religion  

And follows the principles of his religion 

in short — A Staunch — hardcore follower / believer / supporter of Hinduism.