What is the word for first son in Japanese?

There isn't one single word that perfectly translates to "first son" in Japanese. The way it's expressed depends on the context. You would typically say something like:

* 長男 (ちょうなん): This literally means "first son" and is the most common and direct way to say it.

You might also hear phrases like:

* 一番上の息子 (いちばんうえのむすこ): This means "the oldest son." While not exclusively referring to the *firstborn*, it often implies it.

The best choice depends on the nuance you want to convey. For most situations, 長男 (ちょうなん) is perfectly appropriate.