"Every Ancient Minhag has a Root and a Source In Israel's Literature"
Who said this?
Why it's the Torah Temimah on P. Vayeshev 38:10 re: a "Minhag Ta'ut" to not mourn a b'chor the first born son!
The Rema YD 374:11 terms this Minhag a "Minhag Ta'ut" - flowing from a Shu"t Rivash. And as the TT says - all the acharonim remain silent
Nevertheless, Targum Yonatan [TY], is the apparent source for this - apparently because Yehudah named his SECOND son ONAN
And as the TT says:
«It is known that all of his [TY] words flow from Braitot and Midrashim»
Of course we might not actually pasqen like this Targum Yonatan, but at least it has a solid Torah Source Text. And we are not surprised to learn that the TT is the son of the Aruch haShulchan who frequently searches for m'qorot for questionable minhaggim.
We should also distinguish between ANY "Minhag b'alma" and a Minhag Attiq. I, would presume a Minhag Attiq has greater peer approval and it's source might have grown more obscure over time.