It's not a buzzword and existed long before it was appropriated by religious apologists.
It just means excessive trust in the methods, accuracy and scope of the sciences. Nothing sinister or "anti-science". Nothing pro-supernatural or pro-religion about it.
It goes back to criticisms of people like Auguste Comte who thought you could create a "science of everything". It could also be applied to Hegelian/Marxist 'scientific' theories of history, or those who think you can create a scientific morality, or those who put too much trust in softer sciences with very high error rates (psychology, economics, medicine, etc), or those who treat the real world practices of scientists as if they are overwhelmingly in accordance to the normative aims of idealised science. People may also try to apply scientific methods beyond their functional scope resulting in incorrect or misleading data or accept oversimplified metrics simply because they are measurable and thus "scientific" .
Any rational sceptic should be concerned about scientism, but the tendency is to pretend it doesn't exist just because some apologists misuse the term.
It's a bit like pretending fascists don't exist, just because the term is abused to score political points.