An old proverb/phrase mocking the eldest son as being foolish or inept; an archaic, regional expression.
昔の村芝居では『総領の甚六』という役がのろまとして演じられた。
In old village plays, a role called 'Sōryō no Jinroku' was performed as the simple-minded character.
彼は冗談で『総領の甚六だな』とからかわれたが、いまはあまり使われない表現だ。
He was teased jokingly as 'Sōryō no Jinroku,' but that expression isn't used much today.