三男 (sannan) means “third son” | MotiTan Japanese Dictionary

More examples

家族の中で、彼は三男として育ちました。ka zoku no naka de kare wa san nan to shi te soda chi ma shi ta

He grew up as the third son in his family.

Describes a son who ranks third when the male children in a family are counted by birth order.

三男坊sannanbō: An informal term for the son who is third among a family's male children.

同じ親から生まれた男子のうち、出生順が三番目の者。ona ji oya ka ra u ma re ta dan shi no u chi shus shō jun ga san ban me no mono

三男 (sannan) is a Japanese compound of 三 (san, "three") and 男 (nan, "male"), forming a family-order term for the third son.

In family introductions, "third son" identifies a son's position among his brothers rather than his position among all the children.