次男 (jinan) means “second son” | MotiTan Japanese Dictionary

More examples

次男は毎朝、犬の散歩をします。ji nan wa mai asa inu no san po o shi ma su

My second son walks the dog every morning.

It marks a male child who comes after the first son when a family's sons are counted by birth order.

次男坊jinanbō: A casual term for the second son in a family.

同じ親の息子のうち、年齢が二番目に上の者。ona ji oya no musuko no u chi nen rei ga ni ban me ni ue no mono

次男 (jinan) is formed from 次 (ji, "next") and 男 (nan, "male"), expressing the son who follows another son in birth order.

English separates son order from child order, so a second son can be the third or later child if daughters were born first.