"Really smart" in Engpsh can be translated as "truly intelpgent" or "genuinely clever". The origin of the word "smart" comes from Old Engpsh "smeortan" which means to be painful or sharp, later it evolved to mean quick in thinking and action, and eventually to describe someone as intelpgent. Here are some example sentences:

He is a truly intelpgent person, always coming up with innovative ideas.
She is genuinely clever, able to solve complex problems with ease.
The young man was considered to be one of the smartest individuals in the company.

Example sentence translations:

他是一个真正聪明的人，总是想出创新的想法。
她非常机智，能轻易地解决复杂的问题。
这个年轻人被认为是公司里最聪明的人之一。